sneakyimp wrote:Might they go into the main error log?
Any general SMTP errors that prevent the message from being accepted for delivery will generate PHP error messages (E_WARNING, E_ERROR, something along those lines - best to have error_reporting set to E_ALL at all times).
However, note that if the problem occurs after the message is accepted for delivery by the MTA, then it should be obvious that there is no way that PHP can detect this let alone log anything about it.