Debug smtp connection - linux

Question

I am using the perl module Net::Smtp_auth to send a mail to myself using the web.de SMTP server on port 25 (no encryption). It works well my computer. But I would like to use the same script to send notification messages about finished jobs on a different machine where I don't have root privileges. On this machine I get a timeout for the connection. I wonder in which way the connection is blocked, what might be the reason to block it (prevent spamers?) and if there might be a way to circumvent the problem. I have some linux tools available but no nmap.
Might the connection be influenced by proxy settings and, if yes, how do I tell it to SMTP_auth?
The perl script is basically the SMTP_auth example from cpan. But I do not expect that it is relevant here:
#!/usr/bin/perl -w
use Net::SMTP_auth;
$message=$ARGV[0];
$smtp = Net::SMTP_auth->new('smtp.web.de') or die "Failed to open SMTP connection: $!";
$smtp->auth('CRAM-MD5', 'adress', 'password');
$smtp->mail('adress#web.de');
$smtp->to('adress#web.de');
$smtp->data();
$smtp->datasend("To: adress\#web.de\n");
$smtp->datasend("From: adress\#web.de\n");
$smtp->datasend("\n");
$smtp->datasend("$message\n");
$smtp->dataend();
$smtp->quit;

Answer 1

If the other machines are in a different network than your home machine, it may well be that the ISP there is blocking outgoing port 25 to anything but their own SMTP servers, as that is a common spam-mitigation technique. The established convention is that for outgoing mail across networks, one should use port 587 (the SMTP submission-only port) instead.
An easy way to verify if the problem is your program or not is to try to telnet to that host and port. If telnet doesn't connect but other services do, your ISP is filtering port 25.