Q: Mail ALWAYS dropping off IMAP server connection

I'm desperately seeking help!

When connected to the wifi at home, my Mail app is constantly dropping out - often the message that comes up when I run the mail connection doctor says that it:

"Could not connect to this IMAP server. Check your network connection and that you entered the correct information in the Account preferences. Also verify that the server supports SSL. If it does not, deselect the "Use SSL" checkbox in the Advanced tab of the Account preferences."

Of course I have tried all this, constantly, and to no avail.

However, if I tether to my hotspot on my phone I have no issues!

Why oh why would this be? This has been doing my nut for MONTHS and I'd really appreciate any advice!