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7 Debugging

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.