Unable to integrate email for client

I am simply unable to integrate email for clients.

Depending on the entries I make I get various error messages -
For example, although I am (fairly) confident I have the correct settings for Pop3 (tried various settings for server name - domain name, mailservername, IP address etc) I get "Mail file "INBOX" does not exist". However, when I deliberately put in a wrong password I get the same message si I am not even sure if it is connecting ok.

If I try using IMAP I get "IMAP error:{CLOSED IMAP connection broken . . . )

If I try using a secure connection I get an additional error - "Imap error: ESMTP Exim 4.69..."

The email addresses used work fine when tested in other clients.

Any thoughts welcome.

Thanks