IMAP config, unable to receive mails in eM Client with force usage of SSL

I had purchased a MX route powered mailbox from a reseller, and I have competed the DNS setup in my registerar website for my domain name. When I tried configuring my eM Client on windows, the SMTP works fine on port 587 whereas IMAP doesnt get connected on port 993. I later checked the settings and found these

When tried selecting the legacy port option, IMAP works fine and I’m able to receive messages

I would appreaciate if someone can explain me why it hadn’t worked on the default case and required some special port.