Never heard of that, but it's NOT uncommon to have numbers in a email-address and specifically from a internet free email provider like gmail. Heck you will block a lot of emails if you actually did this ;)
I wonder if you have a incoming policy with a LDAP control to do this tho. This might be doable let's in a organization and address verifications for OUTBOUND but for incoming that would be strange approach? But typically ldap is done for recipient verification and to set specific receipient AS/AV profiles and controls from experience.