Have Mailcow set up for several domains. Everything working nicely. I can connect my laptop via smtp and send emails to the world from all domains.
Now I have that web application on a cloud server. The server’s dns name belongs to one of my domains. There is a dedicated mailbox in this domain which works. The application sends out mail by authenticating with the credentials of that mailbox (using javax.mail…). Mail with a destination of a mailbox of any one of my domains gets through, but when I send mail to a 3rd party domain like hotmail.com or any such Mailcow accepts the connection but then complains:
250 2.1.0 Ok
554 5.7.1 <firstname.lastname@example.org>: Relay access denied
DEBUG SMTP: Invalid Addresses
DEBUG SMTP: email@example.com
DEBUG SMTP: Sending failed because of invalid destination addresses
The sender firstname.lastname@example.org exists and has the credentials my application uses.
What am I missing here?
THX a lot for any help