Email sending is main feature of any hosting account that must have feature whether on linux or on windows OS. Most of the users use desktop clients to send or receive their emails, and yes that is the easiest way to communicate with your clients. Okay, lets come to the main topic, if you’re web hosting provider and using cPanel/WHM hosting panel to provide web hosting accounts to your customers then you should read this article. It will save your 24 to 48 hours for sure.
Recently cPanel/WHM has released its latest version that is 68, they made some major updates one of them i have found is updates to SSL and TLS authentications. They introduced new mechanisms and algorithms for the authentication system used for SMTP server (dovecot, exim). What it does? it just disables old authentication system that breaks the SMTP authentication functionality with old email clients such as Outlook 2007 & Outlook 2010. So at the end your clients will not be able to connect to your outgoing email server (SMTP) if they’re using Outlook 2007 & 2010 versions.
First of all i checked mail logs for exim: