Redirect to external mail server gives timeout error
I have a redirect rule that redirects some emails to an external address. The redirected email comes back however with the error message "conversation with DESTINATION-EMAIL-SERVER [DESTINATION-IP-ADDRESS] timed out while receiving the initial server greeting". I also get this error when just sending an email to the same external address (so without rules) using the mailbox.org servers.
When using non mailbox.org servers, everything just works fine. Messages arrive without issues. I tested this with three (3) different email services (smtp servers from different companies/organisations) on three (3) different networks (different access).
The strange thing is that before, everything just worked. There were no config changes to the DESTINATION-EMAIL-SERVER in the mean time.
Did somebody else encounter this? Any solutions?
My mailbox support ticket is: 2016112910001582. They see it as a problem on the receiving end (so the destination email service).
Any help appreciated.