The most common cause is your ISP. More and more ISPs are starting to restrict email sent out of their connection through any outgoing mail server but theirs. So it's very possible that your email set up in Outlook or Outlook Express will work perfectly, then all of a sudden you can't send mail.
You can try changing the SMTP port to 225, if that doesn't work you'll have to change your outgoing mailserver to the ISP's outgoing mailserver. This will not change your MM email address, in fact they'll never know how you send mail through, they only see your return address.
Use the following to set your Outgoing Mail (SMTP) to your ISPs:
1. Open Outlook Express
2. Click on Tools
3. Click on Accounts
4. Click on the Mail tab
5. Click on the account name
6. Click on Properties
7. Click on the Servers tab
8. Insert the Outgoing Mail Server (SMTP Server). This should be set to SMTP server of your ISP ( example: mail.bellsouth.com but you'll have to check with the ISP to make sure)
9. Then down under "Out Going Mail Server" put a check in the box ( if it's not there already )
10. Click the "Settings" button next to it
11. In "Logon Information" check the " Log on Using" radio button and put your ISP user name and password in the fields available.
12. Put a check in "Remember my password"
13. Click OK
14. Click OK again ( on the remaining open window )
15. Then close the remaining window ( if it allows you to click ok or apply do that first )