A working custom-domain identity that receives through your existing inbox and sends through authenticated MailerZ SMTP.
Before you begin
A verified domain and an alias that forwards successfully.
The SMTP host, port, username and generated password shown in your MailerZ dashboard.
Access to the destination inbox so you can confirm verification messages.
Step-by-step
Keep your existing incoming account
Use the IMAP or POP account of the mailbox that receives MailerZ forwards. MailerZ itself does not provide an incoming mailbox.
Create an outgoing server
Open Account Settings → Outgoing Server (SMTP) and add the MailerZ host, secure port, encryption method, username and generated password shown in your dashboard.
Add an identity
Under the destination account, open Manage Identities and add your custom-domain address and display name.
Associate the SMTP server
Choose the MailerZ outgoing server for that identity, save, then restart compose if the identity does not appear immediately.
Run an external test
Send to a mailbox at a different provider and reply back to verify both directions.
Copy credentials into the correct fields
This diagram demonstrates field placement and sequence. Use the unique values in your MailerZ dashboard; never copy credentials or verification tokens from a public tutorial.
Field-by-field reference
Treat this as a translation layer between MailerZ and the provider interface.
Your verified custom-domain aliasThe identity recipients should see.Host shown in MailerZSends the message through the authenticated MailerZ relay.Secure port shown in MailerZMust match the selected encryption mode.TLS or STARTTLS from dashboardProtects credentials and message submission in transit.Generated SMTP usernameIdentifies the workspace credential—not necessarily the email address.Generated SMTP passwordAuthorizes submission; rotate it if exposed.Verify the setup
Thunderbird prompts for and stores the dedicated SMTP credential.
The custom identity appears in the From selector.
Forward and reply both complete successfully.
Avoid these issues
Authentication failures usually indicate a copied username/password or encryption mismatch.
Do not reuse an older SMTP password after rotating it in MailerZ.
A default SMTP server can override the identity-specific server if it is not explicitly selected.
Common problems and exact next actions
Authentication failed
Likely cause: The username, password, port or encryption mode does not match the current MailerZ credential.
Next action: Re-copy all four values. If the password may be stale, rotate it and update only this client.
Message sends from the inbox address
Likely cause: The client selected its default identity or default outgoing server.
Next action: Show the From selector, choose the custom-domain identity, and confirm that identity is linked to MailerZ SMTP.
Verification message never arrives
Likely cause: The alias or destination is not forwarding successfully yet.
Next action: Test inbound forwarding first from another provider, then repeat the client’s ownership-verification step.
Provider interface details can change. Check Mozilla Support
Configure one address, then prove the full loop.
Receive a message, reply through SMTP, and confirm the expected From address before moving your whole domain.