Sometimes delivery fails, often with the generic error message ‘Sending failed; SMTP protocol error’. Enabling one or both of the following variables and inspecting a trace buffer will often give clues to the reason for the error.
smtpmail-debug-info
¶The variable smtpmail-debug-info
controls whether to print
the SMTP protocol exchange in the minibuffer, and retain the entire
exchange in a buffer *trace of SMTP session to server*,
where server is the name of the mail server to which you send
mail.
smtpmail-debug-verb
¶The variable smtpmail-debug-verb
controls whether to send the
VERB
token to the server. The VERB
server instructs the
server to be more verbose, and often also to attempt final delivery
while your SMTP session is still running. It is usually only useful
together with smtpmail-debug-info
. Note that this may cause
mail delivery to take considerable time if the final destination
cannot accept mail.