Recently, and mainly to fight SPAM, email servers have changed to require SMTP authentication for outgoing mail. Often the server address settings are correct, and everything has been working up to now, but suddenly stop. In Outlook Express, for example, go to Tools, Accounts, then the Properties of the affected account, and on the Servers page, under Outgoing Mail Server, there is a checkbox for SMTP Authentication, put a checkmark in the box. This was a monthly occurrence for me. I could never remember what their tech support told me to do last time, so I would have to call again. I'd fix it, and then the proble would recur with no changes made on my part. I came to realize there were as many different explanations and fixes as phone calls I made. I fixed it by changing ISP's.