For security reasons, Mailix only allows certain sender email addresses for messages sent via Mailix.

Which sending addresses are allowed without any further settings?

Every owner of a mailbox can send messages with the following sender addresses without any further setup steps:

• with the e-mail address of the mailbox
• with all alias addresses assigned to this mailbox

Which sender addresses can optionally be allowed additionally?

The e-mail administrator can allow individual mailboxes to send messages with the following sender addresses by setting this accordingly for the mailbox in question in the Mailix UI:

• with any other configured e-mail address of the domain
• with any other configured e-mail address of any alias domain
• with any sender (existing or not existing) in the domain
• with any sender (existing or not existing) in any alias domain

Which sender addresses are not possible under any circumstances?

It is not possible to use a sender e-mail address of any domain not set up for you at Mailix.

See also:
Mailix Documentation: Error 553 - Sender rejected