>Oh ok.. well then another question is that whether I could rely on
getmxrr atleast to figure out whether the doman of the email address is
valid ? Is it a fully reliable means ?
If a domain has no MX record and it has no A record, a mail server
will not have any place to try to deliver it. In that sense, there
are no false negatives (the case of the domain registrar accidentally
taking down the domain because someone else's check bounced comes
under the heading of TRUE negative, at least for the short time
while the mess is straightened out, as real mail servers will fail
the mail also).
You can get TEMPORARY failures of DNS (timeouts on servers answering,
servers down, fiber cuts, servers not authoritative for stuff they
should be, etc.) but these errors can be distinguished from the
permanent errors (no such record here, final answer). I think PHP
lets you distinguish the errors.
Trying to send a bounce message, stopping short of sending a body,
can have plenty of false positives (server might not check name at
SMTP time), but there are enough true negatives (many of them DO
check) to make the check worth it beyond just checking the mx/a
records on the domain. You can also get "false" negatives: the
server accepts *NO* bounce messages period. I question whether you
want to accept mail from such a domain at all.
Gordon L. Burditt