Apple Mail SSL or TLS - El Capitan

Before we begin, yes, I understand there are up-to-date versions of OSX but this particular MacPro (early 2008!) is still a complete workhorse and I'm not going to change it just yet, it's just not going beyond 10.11.6

...so, my issue is, I own a small hosting company and have just switched providers but their setup has completely affected all of my clients' email accounts, including my own using Apple Mail.

For the past umpteen years, my own port settings in Apple Mail have been IMAP 993 & 465 but I can now only get anything to work using 143 (non-SSL) & 587 (SSL). A very strange mix indeed.

After going back and forth for the past three weeks since migration, my new hosting provider is now telling me that TLS is taking or has taken over from SSL, in which case I am having trouble because all I can find are two TLS Certificate options: none or com.apple

Without updating OSX versions (and I can't on this particular machine), is there a way to get to 993 & 465 SSL (or TLS)?

FYI - One of my clients uses Thunderbird and he changed settings to TLS and hasn't had any issues since. Is my problem OSX related, Apple Mail related or can it be fixed using my current 10.11.6?

(as a related issue, I have angry clients currently unable to use their old settings in Windows Outlook who are still having to use webmail whilst I sought out this issue, which means it's not just a Mac thing).

My host sent me this link, if it helps - How to Adjust Cipher Protocols - cPanel Knowledge Base - cPanel Documentation

Thanks in advance for any help and suggestions : )