"Can't call method "auth" on an undefined value at..." after many successful runs - perl

Question

i have a perl app that is supposed to send emails to a massive number of recipients. It seems to work ok, but after about 9K emails it fails with:
Can't call method "auth" on an undefined value at...
In the code I see:
# Open a connection to the SendGrid mail server
my $smtp = Net::SMTP->new('smtp.xyz.net', Port=> 25, Hello=>$DOMAIN);
# Authenticate
my $code = $smtp->auth($USERNAME, $PASSWORD);

Answer 1

The Net::SMTP constructor returns undef if there's a problem (e.g. it's unable to connect to port 25 on smtp.xyz.net). You aren't checking for that, and when you try to call a method on undef, you get the error message you mentioned.
my $smtp = Net::SMTP->new('smtp.xyz.net', Port=> 25, Hello=>$DOMAIN)
or die "Failed to open SMTP connection: $!";
may give you more information. (Although it's not necessarily a socket error, so $! may not contain anything useful.)

Answer 2

The Net::SMTP documentation says that when a method fails it returns undef. So I expect your method call failed.
You might be able to get more information by enabling the Debug => 1 flag in the Net::SMTP constructor.
You will want to detect that your method call failed, and possibly retry it after a short wait.
# Open a connection to the SendGrid mail server
my $smtp = Net::SMTP->new('smtp.xyz.net', Port=> 25, Hello=>$DOMAIN, Debug=>1);
die "Failed to make connection" unless ($smtp);
# Authenticate
my $code = $smtp->auth($USERNAME, $PASSWORD);

Answer 3

You could change it to retry in increasing intervals
something like this:
my $retry = 10; # in seconds;
my $smtp = Net::SMTP->new('smtp.xyz.net', Port=> 25, Hello=>$DOMAIN);
while (not defined $smtp) {
if ($retry > 300) {
die "could not connect to smtp server, giving up";
else {
print "could not connect to smtp, retrying in $retry seconds\n";
}
sleep ($retry);
$smtp = Net::SMTP->new('smtp.xyz.net', Port=> 25, Hello=>$DOMAIN);
$retry *= 2;
}
# Authenticate
my $code = $smtp->auth($USERNAME, $PASSWORD);

Answer 4

This happens mainly if you have the wrong mailbox. Check if smtp.xyz.net is the correct smtp mailbox or it could even be mail.xyz.net. That kind of error occurs if auth is not able to work with the value of 'host' given.