Mailgun validations
Using Jamroom
From memory at one point in time mailgun offered a free service and the free service came with 100 (I think?) free email validations each month. Then a charge was optional to validate more than that. By 'validate' I believe it means "Attempt to determine if an email sent to this address will be delivered."
I think the purpose of it was for people with email lists to eliminate bad addresses because sending is charged per-mail.
Something like that.