v18: mail over port 587 not being delivered

Today i upgraded my XG 17.5.9 to XG 18.0.0. 
Optimized and cleaned up all the migration rules.

Now everything seems to work fine except for one thing. The mailserver i am using cannot connect to a relay host on port 587.
The message it returns in  the log is "Status: 5.7.0 538 5.7.11 Encryption required. Please reconnect with TLS enabled."

I've checked the knowledge base but am not able to resolve this.
Hope anyone has any ideas to help me.

Thnx, Peter-Paul