I have a server with Exim4 It manage the domain example.com
A desktop user which running Thunderbird, is using the SMTP of his ISP (it don't using the server's smtp for some pertinent reasons).
The SMTP outgoing server is authentified by SSL, on 587 port.
When the user, owner of the mailbox firstname.lastname@example.org, send a email to anybody, it's working without problem.
But if email@example.com try to send a message to firstname.lastname@example.org, it received an error message like this :
This is the mail system at host slow1-d.mail.gandi.net. I'm sorry to have to inform you that your message could not be delivered to one or more recipients. The mail system <email@example.com>: host domain.com[213.xxx.xxx.xxx] said: 550 smtp auth requried
Yes, the writing error is real on "requried" ;)
And on exim4 log I have this :
2016-04-13 12:06:42 H=slow1-d.mail.gandi.net [217.xxx.xxx.xxx] F=<firstname.lastname@example.org> rejected RCPT <email@example.com>: smtp auth requried
What's going wrong ?