We have totally disabled SSL/TLS v. 1.0 and 1.1 for email due to security issues. The protocols were deprecated quite some time ago, but many of our customers were using old mail clients or operating systems (e.g., Outlook 2010/2016, Win7) that would only connect using those protocols. Please update your mail client.
If customers cannot connect, please refer to OS manufacturers' support for background info:
Microsoft/O365
https://docs.microsoft.com/en-us/security/engineering/solving-tls1-problem
https://docs.microsoft.com/en-us/mem/configmgr/core/plan-design/security/enable-tls-1-2-client
https://docs.microsoft.com/en-us/archive/blogs/schrimsher/enabling-tls-1-1-and-1-2-in-outlook-on-windows-7
https://docs.microsoft.com/en-us/microsoft-365/compliance/prepare-tls-1.2-in-office-365?view=o365-worldwide
Apple/Mac
https://developer.apple.com/news/?id=bv8ur34d
https://www.greengeeks.com/tutorials/how-to-enable-tls-1-1-and-1-2-on-mac-mail/
