smtp-callout is a small library to check whether an email address is
valid.  It does this by contacting the recipient's SMTP server and
sees whether it rejects the address in question or not.  No actual
mail is delivered to the address in question.

There are many caveats here: Some MTAs will say they accepts mail to
all recipients, but then do the "real" check after data has been sent
(and the MTA checks whether the contents look like spam or the like).

Some MTAs will respond with a "451" code, which means "send the mail
again later", and this is usually because greylisting is in effect.
It may or may not have checked whether the recipient exists first.
Some MTAs respond with a "451" even if they never accept mail for that
address.

Some MTAs will reject the email if the host you're running this on
isn't listed in the SPF records for the "MAIL FROM" address.  In that
case, you can add the host you're running this on to the SPF record
for that address:

"v=spf1 mx a [...] ip4:172.217.20.46 -all"

The actions this function takes may remind some MTAs of actions
spammers take, and you may end up in RBLs as a suspected spammer.
This is very much the case if the address you're checking doesn't
actually exist.

Don't use this if you aren't aware of the possible consequences.