SMTP issue

We have upgraded our exchange environment from 2013 to 2019. Now we have co-existence environment with 2013 and 2019.

We are using f5 as load balancer for email.

The problem is that Outlook & web over http is working fine.

But secure SMTP (TLS) service for pop & Imap is not working through f5 load balancer. It is working directly through the server.

What may be the issue f5 virtual server configuration.

I request a kind support from this community.

Regards

Ashraf