send-email returns non-NIL upon success
According to the documentation:
(cl-smtp:send-email host from to subject message
&key (port 25) cc bcc reply-to extra-headers html-message
authentication attachments (buffer-size 256) ssl)
Send email.
Returns nil or error with message
... but in practice, it's returning a non-NIL result upon success:
CL-USER> (cl-smtp:send-email "smtp.tpg.com.au" "duncan@example.com" "duncan@example.com" "Test" "Test")
("2.0.0 mail15.tpgi.com.au closing connection")