bei host.domain.tld:143 => Unsupported authencation mechanism
I think you need check your IMAP-server settings for authenticate mechanism (PLAIN, LOGIN).
/etc/dovecot/conf.d/10-auth.conf with
# Space separated list of wanted authentication mechanisms:
# plain login digest-md5 cram-md5 ntlm rpa apop anonymous gssapi otp
# gss-spnego
# NOTE: See also disable_plaintext_auth setting.
auth_mechanisms = plain login
bei tls://host.domain.tld:143 => unable to negotiate TLS
and usually TLS work with 465 or 587 ports (tls://host.domain.tld:465 or tls://host.domain.tld:587)
143 - for clear imap, without TLS or SSL