iOS / MacOS issue: system does not allow the user to "trust" a SSL/TLS certificate after renewal
There are two possible solutions:
Solution 1. Recreate mail accounts devices
Remove mail account from iOS / MacOS device.
Re-create email account on iOS / MacOS device.
Solution 2. Manually install and allow using required SSL certificates from device settings:
Get the certificate. The certificate can be exported from the browser to a .cer file:
Upload the .cer file on iOS device through email, Safari browser or File Sharing and install it by clicking/tapping on the uploaded file.
Set up the email account.
If more information is needed on the certificate, it can be found in: Settings > General > Profile.
Note: Interfaces on different versions of iOS / MacOS may vary.