I've fielded a number of questions about this over the past couple of weeks, so I thought it would be worth writing about. While I generally prefer to write about policy issues these days, I'm not above a good nerd piece.
If you experienced an abrupt disruption in your ability to send email from home, it's likely that you're the victim of SMTP filtering by your home ISP. Verizon, Comcast, and other ISPs implement SMTP filtering to stop spammers from taking over computers on their networks and using them to send SPAM. If you find that you can no longer send mail from home because of this, there are a couple of solutions.
1) Use an alternate port, if your server supports it -- the default port for SMTP (sending mail) is 25. ISPs block messages by filtering out any traffic bound for port 25. Many mail servers also support ports 587 and 465 for sending mail. Try going into your SMTP settings and changing your port to 587; see if that fixes it. If not, try 465.
2) Use your ISP's SMTP server. Most ISPs will provide you with an email account (which you may or may not use), which is useful in bypassing this issue, as ISPs will not filter their own servers. Properly configured, you'll be able to send mail using their servers and have your mail appear to come from your usual email address.
Also see my previous post on port 587 -- the most popular post on the site, according to Google Analytics.
Hope this helps!