Figured this out after learning that simply having a domain alias that forwards to the corresponding mailbox at another domain doesn’t allow the mailbox to send as the email address it successfully receives for.
- Login to Mail-in-a-Box control panel.
- Go to Mail > Aliases.
- Create a domain alias for
secondary.tld
that forwards to@primary.tld
. - Create an alias in Roundcube > Settings > Identities. Only required field is email.
- Compose an email in Roundcube. Select the alias in the field From dropdown menu. Write “Test” in field Subject. Write an email address in field To.
- Send.
SMTP Error (553): Failed to add recipient “[target@email.address]” (5.7.1 <[alias]@[secondary.tld]>: Sender address rejected: not owned by user [current_account]@[primary.tld]).
- Login to Mail-in-a-Box control panel.
- Go to Mail > Aliases.
- Edit domain alias.
- Choose “I’ll enter the mail users that can send mail claiming to be from any address on the alias domain.”.
- Write in the field that appears the email address of the account which you want to use to send as the alias from.
- Press the button Update to save.
- Send the email in Roundcube.