Outbound mail issues: SMTP Error: Could not connect to SMTP host


on a new SuiteCRM installation Version 7.11.7 I have problems to setup emails via smtp in Admin > outbound emails,

Some mails suddenly worked after many failed tries with correct logins.

But I it does not work with specific mails hosted on a exchange server which are really needed in SuiteCRM. For inbound they are working fine,

But when I try to set the same mail for outbound / smtp I get the following error messeges (I anonymized the different server adresses with placeholders):

I have searched the forum and web for a solution, but nothing worked for me - in the following forum post someone struggled with the same error and fixed it with a ssl cert install on his site (but my site already runs with a Lets encrypt cert):

SMTP Error: Could not connect to SMTP host.

Does anyone have an idea?

Could my current PHP Version be the reason for this issue? The System Check in > Admin > Upgrade Wizard told me the current php version is 7.0.16. And the recommend PHP-Version are 7.1.0 or higher.

I am really struggling with this - and it would great if someone out there could help!

Thanks a lot in advance - best regards

Did you checked outbound port is allowed?
Check your outbound traffic allow status.