Email help

For months, I have had a recurring email problem. I have spent hours with tech support from Xfinity and Apple, and any fix is temporary. First, I can go on line to Xfinity and get my mail and send mail. Second, using the Mail app on my Apple products, will not always connect.The report says, "Trying to log in to this IMAP account failed. Verify the user name and password".  Third, most of the time can receive mail, but the I can't send mail, or respond to mail sent to me. The message is "Could not connect to SMTP server. Check your network connections, and that you entered the correct information in account preferences.Also verify that the server supports SSL.  If it does not, deselect the "use TLS/SSL"checkbox in the Server Settings tab of Account preferences." Lots of the tech support think that they have this fix, only to fail again. Apple says that because it is a problem across several Apple devices, and the fact that it works some of the time shows that it is not a problem with the computers, iPads , iPhones etc . Please Help. I am hoping that the Yahoo switch will help me. It can't come soon enough.