I also get this error when sending a personal message.
Could it be that this error occurs whenever the system tries to send an email. For example, when someone has asked to be notified of new personal messages or replies to a certain thread?
I think it must be something like that. This also explains why this error doesn’t happen every time one posts: only if someone is \"following\" the thread, this error will happen.
Please look into this. Maybe you just need to update the smtp settings, or simply restart a service or something.