Don't know how hard it can be to set an imap-account but anyway I'm having issues.
I am able to receive emails and I can see them in my inbox. But when I reply to a received email, the message is never fully send.

"Sending message (100% complete)"
but the message is still in the 'outbox'-folder and is not received by the receiving party.

I am using plain authentication... no ssl or tls to receive mail. Same conditions for outgoing mail.

Server type : smtp
server : mail.server.tld
server requires authentication
no encryption
authentication type : login/plain both not working
(check for supported types takes forever)

Does anyone know how to debug ?!

If I'm not wrong, you don't use IMAP(s) for *sending* mail, you use it for *getting* mail.

Talking about mail, "getting" and "sending" are two separate beasts.

For the "sending" part, you should use plain SMTP, or encrypted SMTP; and for the "getting" part, you use POP, IMAP, or the encrypted versions of the former two.

So, you should check the SMTP part, not the IMAP part. Perhaps setting the right address and port to connect

As an example: to use Gmail, you must set the SMTP server adress as "smtp.gmail.com:587" and set "TLS" too. I had a google a little to find this.

Other thing you can try: "telnet mail.server.tld 25", and send some SMTP raw text, and see if it responds. That way you can be sure the mail server is really up and running.

My hosting provider says :

incoming mail server: mail.domain.tld
incoming mail server: (SSL) x37.alfaservers.com
outgoing E-mail Server: mail.domain.tld (server requires authentication) port 587
outgoing E-mail Server: (SSL) x37.alfaservers.com (server requires authentication) port 465

So for outgoing mail I have :

server type : smtp (of course not imap)
server : x37.alfaservers.com:465
user secure connection : ssl
authentication type : login

When sending a mail :