FAQ 9.17 How do I check a valid mail address?

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9.17: How do I check a valid mail address?

(partly contributed by Aaron Sherman)

This isn't as simple a question as it sounds. There are two parts:

a) How do I verify that an email address is correctly formatted?

b) How do I verify that an email address targets a valid recipient?

Without sending mail to the address and seeing whether there's a human
on the other end to answer you, you cannot fully answer part *b*, but
either the "Email::Valid" or the "RFC::RFC822::Address" module will do
both part *a* and part *b* as far as you can in real-time.

If you want to just check part *a* to see that the address is valid
according to the mail header standard with a simple regular expression,
you can have problems, because there are deliverable addresses that
aren't RFC-2822 (the latest mail header standard) compliant, and
addresses that aren't deliverable which, are compliant. However, the
following will match valid RFC-2822 addresses that do not have comments,
folding whitespace, or any other obsolete or non-essential elements.
This *just* matches the address itself:

my $atom = qr{[a-zA-Z0-9_!#\$\%&'*+/=?\^`{}~|\-]+};
my $dot_atom = qr{$atom(?:\.$atom)*};
my $quoted = qr{"(?:\\[^\r\n]|[^\\"])*"};
my $local = qr{(?:$dot_atom|$quoted)};
my $domain_lit = qr{\[(?:\\\S|[\x21-\x5a\x5e-\x7e])*\]};
my $domain = qr{(?:$dot_atom|$domain_lit)};
my $addr_spec = qr{$local\@$domain};

Just match an address against "/^${addr_spec}$/" to see if it follows
the RFC2822 specification. However, because it is impossible to be sure
that such a correctly formed address is actually the correct way to
reach a particular person or even has a mailbox associated with it, you
must be very careful about how you use this.

Our best advice for verifying a person's mail address is to have them
enter their address twice, just as you normally do to change a password.
This usually weeds out typos. If both versions match, send mail to that
address with a personal message. If you get the message back and they've
followed your directions, you can be reasonably assured that it's real.

A related strategy that's less open to forgery is to give them a PIN
(personal ID number). Record the address and PIN (best that it be a
random one) for later processing. In the mail you send, ask them to
include the PIN in their reply. But if it bounces, or the message is
included via a "vacation" script, it'll be there anyway. So it's best to
ask them to mail back a slight alteration of the PIN, such as with the
characters reversed, one added or subtracted to each digit, etc.



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