I am really confused with your answer:
1 - You really mix-up things about webmail: the point is that webmail acts as IMAP client, reason why it may enable, by default, IMAP port for standard mail, which is preventing Zarafa IMAP gateway to start. Let's Zentyal team react on this.
2 - your telnet on port 143 does show that you access Zarafa IMAP server isn't it?
3 - So far, I don't think there is any so called "Zarafa user DB" is there? Zarafa is configured to rely on LDAP
Difference, from mail standpoint, is that IMAP std mail server (Dovecot) expect login as "address@domain" while Zarafa, authenticating directly against LDAP, expects "login" (your LDAP login).
Even if I never installed Zarafa, please trust me, I'm almost sure of this, just because this is the way components are stacked all together
and back to one you your comment/question:
Is there something else to consider besides assigning IMAP and IMAPS to Zarafa (that means, no check-box is active under Mail Retrival Services...)
this is exactly what I wrote: "ensure mail retrieval services are not activated in the
mail section" which means, in extenso,
NO POP, POPS, IMAP and IMAPS. Did you confuse it with fetchmail?