Not all mails reach the server



Hello everyone,

A few days ago, I installed the latest version of UCS on my server. I installed the mailserver and the ox app suite via the app center. I created the neccessary A, SPF and MX records at namecheap. The setup can send and receive emails to and from my private gmail address.

I thought everything is working fine, when I tried to create a twitter and facebook account with one of the email addresses. The verification / email confirmation mails never reach the mailserver.
The log file /var/log/mail.log shows a connection when I receivce mails from my gmail address, but not in this case.

Is there anything my configuration is missing? Why do Twitter and Facebook mails not reach my my mailserver?

Thanks in advance