I am havin for quite a long time problems with sending erros. And I believe I have detected the possible cause, but I cannot find the remedy for it.
I am connected remotely to the email server of my customer through which I sent emails with their domain.
All emails whose recipients are for that server are sent. All other emails though which a relayed "outside" this server caus the send error and I have to send it via thunderbird which has exactly the same configuration as sugar crm.

What can cause this behaviour?

Thanks for your help