4.8.7. Error "553 5.7.1 Sender address rejected: not owned by user"

Error "Sender address rejected: not owned by user" when sending an email means that an attempt to send is being made using someone else's or another mailbox. There are two situations when this error may occur:

• The sender is not the mailbox from which the mail is actually sent, but a different mailbox within the same mail domain.
• The sender is the mailbox of another domain.

⚠️ The error will include the mailbox where the authorization was performed and the mailbox that is specified as the sender:

(5.7.1 <admin@example.com>: Sender address rejected: not owned by user mail@example.com)

• admin@example.com — mailbox where authorization is performed.
• mail@example.com — mailbox specified as the sender.

If when sending a message the authorization was performed in the admin@example.com mailbox, but in the "From" header the mail@example.com mailbox (with the same domain) is specified as the sender and an error occurs — it means that the "Allow sending emails from other users within your domain name" option is disabled in the mail domain settings.

Solution options:

• Or do not specify another mailbox as the sender.
• Either enable the option "Allow sending emails from other users within your domain name".

If when sending a message the authorization was performed in the admin@example.com mailbox, but the admin@domain.com mailbox (with another domain) is specified as the sender in the "From" header, then the mailbox with the same domain must be used as the sender. Sending from a mailbox with a different domain is not possible.