Mail issues on newly installed server

Hi All,
On this recently installed server, has appered issues with mail accounts creation, with Microsoft clients (Outlook etc). MS clients can automatically configure only an IMAP account, not a POP3
Thunderbird has no issues in creation of a POP3 or a IMAP.

My Server: CENTOS 7.4 standard v68.0.13 - cPanel version: 68.0 (build 13)

Note that the domains copied from a already working system with the Transfer tool

What the users get:
Server error: '550 permitted to relay through this server without authentication.'

The Maillog:
/var/log/maillog shows the following (since the clients are unable to login):
dovecot: imap-login: Aborted login (auth failed, 1 attempts in 6 secs) etc..

CPANEL ERROR LOG
/usr/local/cpanel/logs/error_log
Net::SSLeay error left in queue from previous call: βerror:140E0197:SSL routines:SSL_shutdown:shutdown while in initβ
[2017-11-16 10:35:57 +0200] info [cpsrvd] Internal Server Error: "-" 500 The cPanel Server operation timed out at cpsrvd.pl line 535.

I've compared Exim Config. manager and Mailserver configurations in other installations.. but no luck. Also checked carefully the CSF settings, still nothing! :-(

The only difference in this WHM fresh installation is that Mailserver Configuration/SSL Protocols, has been set by default to TLSv1.2
The old servers are using: !SSLv2 !SSLv3

Can anyone help me?
Thank you in advance!

 

Dear All,
Fixed!
For anyone having the same issue:

Go in Exim Configuration Manager
Field:
Require clients to connect with SSL or issue the STARTTLS command before they are allowed to authenticate with the server.
Should be turned "Off"

Cheers!