IMAP problemen met Telenet provider in Belgium

Mails are not (always) synchronizing between my iMac M1 - OS 15.6 ; iPhone 14 Pro - iOS 18.6 ; iMac Pro (2nd gen 11 inch) iPadOS 18.6.

I am using the “standard Apple Mail app. All my settings are 100% to what Telenet is saying or suggesting. Ports for IMAP server and for the SMTP server are ok.

Telenet is claiming that their webmail has no problems, so this problem is an Apple (iOS) problem.

now I read on different fora that the problem could be that Apple is tightening the security measures foor SSL and that Telenet probably is using an older version. I hate to be bounced up and down between Apple and Telenet about who's causing the problem.

Is there anybody who van tell me anything about this??