Assuming it all worked before and you are using ubb 7.5.6+
Also that your host did not change your email server.
There is only two places to check.
1. in the mystuff-Edit preferences.
All the way to the bottom is a check box to elect to be notified.
2. Control Panel » Primary Settings-advanced options.
Use specified SMTP Server:
Normally this is controlled by php.ini. However if you need to send mail through another SMTP server and cannot do this by editing php.ini, you can specify it here.
This field is normally blank.
3. Test that your main email account works since most email functions come from that.
Control Panel » Primary Settings
Board Email Address:
All emails being sent out will come from this address.
My 2 cents.
Something happened to your main email account.
Either your host changed your server settings or your main email account got blacklisted.
Blue Man Group
There is no such thing as stupid questions. Just stupid answers