According to search results, To update, it looks like this issue was resolved adjusting the following values in "WHM Home » Service Configuration » Mailserver Configuration": Maximum Size of a Authentication Process (MB) to 128M Maximum Number of Authentication Processes to 70
You try to send mail (SMTP) and you see an error with an IMAP greeting? Do you have a port set wrong somewhere? Start with your mail client and make sure you're sending on 587 (or maybe 465), not 143, 995 or similar.