Developing on my MacBook, with the MAMP stack installed, attempts by any of my local Drupal sites to send email out to a Drupal user (say, a new account notification) go into the local mail queue and get stuck there. /var/log/mail.log shows "Operation timed out" errors from Postfix, whether sending to an account on my webhost or my Gmail account. telnet to port 25 (using a couple different hosts) also fails with "Operation timed out".
And... now I feel like an idiot, because I had this problem a few years ago with Comcast - my ISP (now RCN) is blocking port 25. Solution:
Took a few tries with telnet [mail|smtp].rcn.[com|net] 25 to find the right server, but I'm all set now...