I've been investigating issues with IMAP on mail.btinternet.com by using the Linux command line (with help from https://seeit.org/2010/01/31/debug-your-imap-server-with-telnet/)
Whatever username and password I try, I get this:
$ openssl s_client -connect mail.btinternet.com:993
[ssl stuff omitted]
* OK IMAP4 PROXY server ready
a001 login fred xyzzy
a001 BAD unrecognized extra arguments to LOGIN command
For comparison, using the Yahoo server, I get this:
$ openssl s_client -connect imap.mail.yahoo.com:993
[ssl stuff omitted]
* OK [CAPABILITY IMAP4rev1 ID MOVE NAMESPACE X-ID-ACLID UIDPLUS LITERAL+ XYMHIGHESTMODSEQ AUTH=plain SASL-IR AUTH=PLAIN AUTH=LOGIN AUTH=XYMLOGIN AUTH=XYMPKI AUTH=XYMCOOKIEB64 AUTH=XOAUTH2 AUTH=OAUTHBEARER] IMAP4rev1 Hello
a001 login fred xyzzy
a001 NO [AUTHORIZATIONFAILED] LOGIN Invalid credentials
which makes more sense.
I've tried asking this question on the BTCare forum, but didn't get a helpful reply.
Is mail.btinternet.com behaving brokenly? It doesn't seem to respond according to the IMAP spec (https://www.rfc-editor.org/rfc/rfc3501).
Use -crlf flag to openssl s_client. By default it doesn't send CRLFs, which does
not conform to the IMAP requirements, and can cause odd behaviour in some servers.
This works for me:
$ openssl s_client -connect mail.btinternet.com:993 -crlf
[snip]
a LOGIN fred xyzzy
* BYE disconnecting
a NO [AUTHENTICATIONFAILED] LOGIN Invalid credentials
closed