If the filter is after the
server providing the form page and the SUBMIT button, yes, the
server received the message. IETF can have a log file before the
filter (SOP), allowing review of what is filtered.
Substantially similar capabilities are present in all of the SMTP
MTA's I'm familiar with. If a message delivery attempt is rejected
early, only envelope information is logged, but if the message was
rejected in error, that's generally sufficient to identify what needs
to be whitelisted.
Ietf mailing list