SMTP relay vs mailbox SMTP: what small teams actually need

Quick answer for smtp relay vs mailbox smtp

Use authenticated SMTP relay when Gmail or Outlook already holds the archive and you only need the domain to leave as the visible From. MailerZ is one product in that class: inbound MX plus paid send-as from verified identities. It is not Google Workspace, not IMAP, and not an open relay. Unhosted or unauthorized recipients get SMTP 550 / 550 5.7.1.

Use mailbox SMTP when the vendor hosts the mailbox you log into. Google Workspace, Microsoft 365, Fastmail, and similar hosts authenticate you as that user. Their SMTP is a door into a store. Microsoft documents device-and-app SMTP against a hosted mailbox in Microsoft Learn — Send email from a device or app using Microsoft 365. That is a different purchase than a delivery-layer credential. Gmail’s client path for a different address is Google Gmail Help — Send mail from a different address — it assumes you already have a working SMTP server allowed to use that address.

Transport still follows IETF RFC 5321 — Simple Mail Transfer Protocol. Envelope commands are not the header block people see. MailerZ may rewrite only the envelope return path with Sender Rewriting Scheme on inbound. Header From, Subject, Date, Message-ID, body, and MIME are never rewritten. Outbound send-as uses the From you approved. SPF, DKIM, and DMARC authorize that domain; they do not turn a relay into a mailbox. See IETF RFC 7208 — Sender Policy Framework (SPF), IETF RFC 6376 — DomainKeys Identified Mail (DKIM), and IETF RFC 7489 — Domain-based Message Authentication, Reporting, and Conformance (DMARC).

Free cannot finish relay send-as. MailerZ Free is one domain, three aliases, one seat, a 14-day store, send-as disabled, SMTP and API disabled, and unrouted mail held or rejected only. Solo is $40 per year only — 15 aliases, 90-day store, 100 outgoing per month, 5 send-as per hour. Starter is $8 monthly or $80 yearly. Business is $19 or $190. Agency is $39 or $390. Unlimited is $99/month or $990/year. Confirm numbers on MailerZ pricing. Those numbers are plan limits, not an inbox-placement promise. The send-as surface is on send and reply as your domain.

Small teams actually need one outbound path they can prove, not two vendors and a “temporary” mailbox SMTP leftover from a Workspace trial. Annual Starter, Business, and Agency billing includes two months free relative to paying monthly for a year. Solo has no monthly option. Campaign blasts do not belong on either hop. Use a campaign platform.

The user problem and the decision criteria

The usual mess is a patchwork. Inbound hits a forwarder. Outbound still uses Gmail’s default server, so recipients see @gmail.com. Someone then enables Workspace SMTP “just for send,” leaves Google MX in DNS, and wonders why mail splits. Someone else pastes their Google password into a random relay form. Someone else asks Outlook to add the domain as an IMAP account because the wizard used the word SMTP.

SMTP relay vs mailbox SMTP is how you stop that mix. Relay means: authenticate to a delivery layer, submit as an approved domain identity, keep reading mail where you already read mail. Mailbox SMTP means: authenticate as a hosted user, send as that mailbox, usually fetch the same store over IMAP or Exchange. Both can be correct. They are not interchangeable passwords.

Small teams need the cheaper honest hop. If three people already live in Gmail and only hello@ must look official, relay send-as plus named aliases is the job. If those three people need lockable hosted mailboxes and Calendar as the company book, mailbox SMTP comes with the suite you should have bought anyway.

Relay is a good fit when the domain is an identity, the store already exists, and you can tolerate published send limits. Mailbox SMTP is a good fit when the host is the store. Relay is a poor fit when you want MailerZ to promise Primary-tab placement, when you need a lockable hosted mailbox per leaver, or when you treat “relay” as an open door for purchased lists.

Apps and scanners add a third confusion. A copier “SMTP relay” field often wants mailbox SMTP against Microsoft 365. A SaaS “custom SMTP” field often wants a relay credential. Putting the MailerZ password in a device that also expects IMAP is how small teams spend Friday night. Read the vendor’s field labels. If they ask for a mailbox login, that is mailbox SMTP. If they ask for host, port, and a generated password for a From you approve, that can be relay.

Shared role addresses do not need mailbox SMTP per name. billing@ is a route inbound and, if you pay, one approved From outbound. Buying a hosted user named billing because a wizard said SMTP is how per-user pricing sneaks back in.

Product surfaces around this choice live on MailerZ features. Field maps live in the MailerZ documentation. This page stays on which SMTP object you are actually buying.

Technical mail flow

Both hops are SMTP conversations. A client connects, authenticates or is rejected, offers MAIL FROM, names RCPT TO, and transfers content. The difference is who you claim to be and which store, if any, sits behind that login. IETF RFC 5321 — Simple Mail Transfer Protocol is the transport standard.

Relay path (delivery-layer send-as)

You create a credential on a paid MailerZ plan: host, secure port, username, generated password. The client — Gmail Send mail as or Outlook manual SMTP — authenticates. MailerZ checks that the From identity is approved, applies hourly and monthly limits, and submits. Invalid credentials stop here. Unhosted or unauthorized recipients get 550. The inbox you read does not move. Inbound still arrived through MX and a forward into Gmail or Outlook, with Header From preserved.

This is why people call it a relay. The message is submitted through MailerZ. MailerZ is allowed to send as the domain because you published the SPF, DKIM, and DMARC values the dashboard shows. It is still not an open relay. “Relay” in operations slang does not mean “anyone may send.”

Mailbox SMTP path

You authenticate as you@hosted-domain on the vendor that stores the mailbox. The vendor submits as that user. Fetching mail is a separate protocol: IMAP, Exchange, or webmail. Small teams hit this path when they buy Workspace or Microsoft 365 and let the client autodiscover the hosted SMTP. That is correct for a hosted store. It is the wrong wizard when MailerZ is only the identity hop.

Microsoft’s device-and-app article is about sending through a Microsoft 365 mailbox, not about MailerZ. If you follow it while MX points at MailerZ, you are mixing stores. Pick one outbound identity path and one inbound MX set.

Inbound is not SMTP send-as

Customers writing to the domain still need MX. MailerZ accepts, stores, and forwards. Unknown recipients are held on Free. Paid catch-all forward is optional. Envelope SRS may rewrite the return path. Header From stays original. None of that creates send-as. Free outgoing of 50 messages per month is not send-as. Do not plan a public From on that number.

What you store

After a successful forward, Gmail or Outlook is the system of record for the copy you read. MailerZ recovery is 14 days on Free and 90 days on paid plans. That window is hop evidence, not a second mailbox and not a reason to treat relay SMTP as IMAP. Legal hold stays in the destination store or another archive you already run.

Step-by-step setup and decision path

Do this in order. Creating an SMTP password before inbound works is how verification mail vanishes and people blame “the relay.”

1. Name the store

   Write the exact Gmail or Outlook address people already search, or admit you need a hosted mailbox. If you need IMAP on the domain, stop and buy hosting. MailerZ will not answer that login.

2. Add one domain and verify it

   Root domain, not a mailbox, not https://. Publish the verification TXT. Receiving and sending stay off until that check passes.

3. Map named aliases and publish one MX set

   Delete leftover Google, Microsoft, or registrar MX. Split records make outbound tests look random because some inbound never arrives.

4. Prove inbound from a different mailbox

   Unique subject, unrelated provider. Confirm Header From and delivery history. Open spam and junk. Do not use Gmail self-send as the only proof.

5. Choose the outbound object

   Existing store plus domain From: paid MailerZ relay SMTP. Hosted mailbox as the store: mailbox SMTP on that host, and MX should match that host — not a hybrid leftover.

6. Create the credential on the plan that allows send-as

   Free stops here. Solo or another paid plan. Copy host, port, encryption, username, generated password. Do not paste your Google or Microsoft account password into that form.

7. Attach it in the client you actually use

   Gmail: Settings → See all settings → Accounts and Import → Send mail as. Outlook: manual SMTP identity, incoming mail stays at the destination mailbox. If the wizard asks for IMAP or Exchange for the custom domain, cancel.

8. Send to a second external inbox

   Check visible From, return hop, and outbound history. Far-side placement is still their filter. Repeat after any client UI rename — the SMTP job does not change when a menu moves.

Failure modes and proof

Most “SMTP is broken” reports are leftover MX, Free asked to send-as, a mailbox password in a relay form, or an Outlook wizard that tried to host mail.

Proof is a sanitized destination header block plus the MailerZ event. Do not send SMTP passwords to support. Do not publish verification tokens. If you need DNS reads, use troubleshooting and DNS diagnostics. Those tools are DNS-only. They do not grab SMTP banners or invent a relay score.

A Workspace trial you cancelled can still publish Google MX and still offer mailbox SMTP in a leftover client profile. Delete the records. Remove the stale outgoing server. Two SMTP identities in one Gmail account is how “sometimes it works” happens.

MailerZ workflow and product boundary

MailerZ is a custom-domain email delivery layer operated by Secuno LLC. Point MX at MailerZ. Mail for a hosted, verified domain can land in Gmail or Outlook. Paid plans add authenticated SMTP so named identities can send and reply. The live site is mailerz.net. The app is mail.mailerz.net.

What MailerZ does on the relay path

• Accept inbound mail for verified domains and named aliases.
• Preserve Header From on the forward.
• Hold unknown recipients on Free; optional paid catch-all forward.
• Store messages 14 days on Free or 90 days on paid plans.
• Authenticate SMTP sessions and send from approved identities on paid plans.
• Refuse unhosted or unauthorized recipients with 550 / 550 5.7.1.
• Record delivery history for inbound and outbound hops.

What MailerZ does not do

• Offer mailbox SMTP, IMAP, or webmail.
• Rewrite header From, Subject, Date, Message-ID, body, or MIME.
• Offer send-as on Free.
• Act as an open relay.
• Promise inbox placement, uptime SLAs, or review counts.
• Claim SOC 2, ISO 27001, or HIPAA. Controls live on Security and Trust Center.
• Send newsletters or purchased lists.
• Replace Workspace mailbox SMTP when you actually need the suite store.

Free is one domain. Solo is three domains. Starter 5 domains / 50 aliases / 5 seats / 2,000 outgoing / 10 per hour. Business 25 / 200 / 25 / 4,000 / 15. Agency 100 / 500 / 50 / 8,000 / 25. Unlimited is $99/month or $990/year. Seats operate the dashboard. They are not mailbox SMTP users.

Cost, alternatives, and trade-offs

Relay SMTP cost is a delivery-layer plan. Free is $0 inbound with no send-as. Solo is $40 per year when you need the hop. Starter, Business, and Agency raise ceilings and can be monthly. Mailbox SMTP cost is a suite seat. Google’s current packaging lives on the Google Workspace — product overview page and changes. Do not treat a blog number as Google’s price list.

Cloudflare Email Routing is inbound routing. It does not become MailerZ leftover-MX handling, stored failed hops, or authenticated send-as because a blog said “SMTP.” ImprovMX is the closest commercial forwarding class; compare live plans for their outbound story. Neither card is a MailerZ claim.

Two months free on annual Starter, Business, and Agency is arithmetic against monthly, not a coupon stacked on Solo. If cash-flow wants monthly relay capacity, Starter at $8 is the published door. Campaign mail still does not belong on that card.

Time is part of cost. An afternoon mixing mailbox SMTP with leftover Google MX costs more than Solo for many three-person teams. Finish inbound. Then create one outbound credential. Two half-configured hops is the expensive week.

Vacation responders stay on the destination. A Gmail vacation responder on Free still leaves as the Gmail identity unless you also paid for relay send-as and told Gmail which From to use. Mailbox SMTP auto-replies use the hosted user. Do not expect a $0 plan to rewrite those messages into the domain.

Agencies sometimes want relay SMTP on every client domain. Free is one domain and has no send-as. Client rosters are an Agency-capacity question. Cheap for your own studio is Solo. Cheap for a roster is the published Agency card, not twenty Free accounts and a leftover mailbox SMTP profile on each laptop.

FAQ

Key takeaways

• SMTP relay vs mailbox SMTP is who you authenticate as, not a logo choice.
• Relay send-as keeps Gmail or Outlook as the store. Mailbox SMTP comes with a hosted login.
• MailerZ is authenticated SMTP from approved identities. It is not IMAP and not an open relay.
• Free has no send-as. Solo is $40 per year when the From must travel.
• Inbound MX and outbound SMTP fail separately. Prove both.
• Do not paste a Google password into a relay form. Do not add IMAP for a MailerZ domain.
• Leftover MX steals verification mail and looks like an SMTP bug.
• Header From stays original on inbound. Envelope SRS is the allowed rewrite.
• MailerZ is not SOC 2 and not a bulk sender. 550 is a control.

Conclusion and next action

If you came here for SMTP relay vs mailbox SMTP, name the store first. Existing Gmail or Outlook plus a domain From is paid relay SMTP after inbound works. A hosted mailbox you log into is mailbox SMTP on that host, with MX to match. Small teams get hurt when they run both leftovers at once.

MailerZ fits the relay job with delivery history and a recovery window you can see. It does not fit hosted mailboxes for every user, certified compliance reports, or campaign-scale sending. Start on Free if receiving is the job. Move to Solo when the From identity has to leave as your domain.

Next action: add one domain, prove inbound from another mailbox, then create one SMTP credential on a paid plan and send to a second external inbox. The register path is one domain, not a mailbox migration.

Review quarterly, or sooner if MailerZ send-as limits, Gmail Send mail as, or mailbox-SMTP vendor docs change. Author: MailerZ editorial, Secuno LLC.