IMAP migration fails with 'Unable to connect to the server'

I've been trying to test out IMAP migration and I keep getting an 'Unable to connect to the server' when I try to add an account. I've done a Google search on this and see it's fairly common, but the normal problem appears to be to do with SSL certificate issues, but that cannot be the case here. I've tried with two accounts, one a Gmail account and the other an account with a large UK ISP, virginmedia.com. Obviously in both cases the SSL certificates are valid (and I double-checked just to be sure at https://www.sslshopper.com/ssl-checker.html#hostname=imap.virginmedia.com:993).

So I don't know why I might be failing to connect. I assume entering the details as below is correct, is it not?

Source User Name: the account name on the IMAP account I want to pull the messages from, e.g. ' joebloggs@gmail.com'

Source Password: the IMAP account password, e.g., 'mygmailpassword'

Destination Email Address: the target account within Zoho, so if I have 'mydomain.com' handled by Zoho, this might be e.g. ' xxx@mydomain.com'