Hello Asin,
can I ask you for a bit more information so we can look more specifically into your problem?
1. What is the version of your os? Click the Apple logo in top left corner --> About this Mac
2. What is the version of your Apple Mail? Open Mail and go in Menu --> Mail --> About Mail
3. What e-mail protocols do you use? Mail --> Menu --> Preferences --> Accounts. It is written beneath your account name in the left panel (exchange, IMAP, POP3) you can also click Advanced and check whether SSL is enabled for your account.
In case you do not want to share that information on this forum, please send it to jakub.bednar@avast.com
Best regards and happy New Year,
Jakub