This error might be referring to the "From" address you specified in the "Outbound Mail Servier Settings" in Console. You will get this error if that address is not valid.
I have also encountered this message. In the event log I was able to go back to when the error first appeared to see what date/time this occurred. Within SQL, I then looked in the tps_user_message tables to see which message corresponded with time (allowing for time zone difference). I then just marked the message as sent within this table.
However, I must admit that the message in question looked fine. The message was generated as an escalation of one of my SLA rules, and other rules had been firing off fine. I haven't been able to replicate it in testing - it has only happened twice in production.
Same with me. I also cannot replicate it in testing. I suspect it related with placeholder in escalations I set in SLA.
Besides, I also always get error log "Object reference not set to an instance of an object" for Outbound Mail. Initially this happen when placeholder have null value. I set default value for every placeholder so when placeholder null it will replace it will default value. I though with this, error object reference will not show again.
But notification will send out by system even though have error object reference.