Mac Mail El Capitain Certificate Rejection

Hi There,

I'm hoping someone might be able to help, before I go crazy.

Ok to put you in the picture, I am in the process of moving a website from one provider to another. My current server provider uses extend and the new provider users cPanel. The website owner is running Mac Mail on El Capitain and can connect to the old server without a problem. The problem we are having is when it comes to connecting to the new mail server it won't go through. According to the tech guys its because Mac Mail is rejecting the certificate.

I have attempted to connect to the server using Mac Mail on High Sierra and this works without any problems at all but on El Capitain its having major problems.

We have tried the following:

• Connecting on different ports
• The cPanel generated mail configuration file
• Connecting using SSL on and Off
• Connecting using Password as Authentication
• Using either IMAP or POP3

The client wants to use IMAP, however they'd like to drag and drop these before commencing the hosting move.

Any help would be great.

Cheers

Rob