Unable to send out emails after upgrade

[Aiko]

I recently upgraded from RC3 to RC4, and now I am unable to send out newsletters and activation emails. I keep getting this error in my log:

Apply Filter: Only show the error messages of this URL
http://sitename/forum/index.php?action=admin;area=regcenter
Apply Filter: Only show the errors with the same message
Unable to send mail to the email address '[email protected]'

I've been checking the forum for help but most lead to this link, which doesn't seem to apply to me: http://www.simplemachines.org/community/index.php?topic=343385.0

Can anyone help?

[JimM]

Can you clarify that no emails are going out or just emails to a specific user or domain?  Have you checked your email settings to make sure that something hasn't changed?