How do I set/change the 'sender' for e.mails from SMF?
I am now using SMF2 rc4, but had the same problem in SMF 1.1.12 - maybe after an upgrade from 1.1.11.
My host (one.com) tells me the reason for only a very limited amount of email notifications get send is most likely a missing "from" address.
Admin --> Server settings --> Webmaster Email Address ;)
The from address ends up being your webmaster email address as Tah Zonemaster mentioned. You might try changing that to a different address to see if it makes a difference.
Moving this to the support forums as it had nothing to do with Install and Upgrade ...