Mail::Checker seems to be unfit for Yahoo
It seems that it is impossible to test existence of yahoo mailbox without sending a test message. Yahoo seems to reject messages to non existing user in reply to "the final dot" in SMTP session.
I have tried to send message to non existing (long random) mailbox with the following result (bounce message):
<d5b980c9018f82c94cefee51193f8a61@yahoo.com>: host
mta5.am0.yahoodns.net[98.138.112.32] said: 554 delivery error: dd This user
doesn't have a yahoo.com account
(d5b980c9018f82c94cefee51193f8a61@yahoo.com) [0] -
mta1516.mail.ne1.yahoo.com (in reply to end of DATA command)