Early this week I called my web hosting provider, BlueHost, to ask to be upgraded from PHP 4 to 5. They graciously offered to move my files to a PHP 5 server at no charge. I was warned, though, that my site might be inaccessible during the move.
Later that day, I saw that the site was running on PHP 5.
One problem: my custom MX records, pointing to the wonderful FastMail email host, were reset to their defaults. People who wrote to me the morning of October 3 had their email bounced back to them.
BlueHost corrected my MX records quickly once I told them. Email flowed properly within an hour or so.
Lesson: anyone with custom MX records should, if altering web host settings, check the MX records immediately thereafter. Easy way to check: MX Lookup Utility.