Favorite modules: Mail Logger
Sending email from a website is a common task - there are password reset emails, welcome emails to new members, order confirmation emails, and sometimes emails letting subscribers know about updated pages, or sharing posts to discussion forums on the site.
There are a lot of ways for email to go astray. If someone doesn't get the email they were expecting, how do you know whether it was sent, but filtered out by the recipient's spam filters, or not sent due to a glitch on the website?