What is the difference between "failure" and "error" replies with
respect to RFC *821?
821 explicitly uses E: and F: replies in
4.3. SEQUENCING OF COMMANDS AND REPLIES
1869 has two subsections about "failure" versus "error":
4.4. Failure response
4.5. Error responses from extended servers
2821 uses only E: in
4.3.2 Command-Reply Sequences
...
success, and "E" for error. Since some servers may generate other
but still refers to "failure" and "error" replies in the text.
For example, it even explicitly distinguishes between those two:
4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO)
...
will give a successful response, a failure response, or an error
response. If the SMTP server, in violation of this specification,
which is probably a leftover from 1869.
If there is some (functional?) difference between "failure" and
"error" replies then this should be explained. Otherwise it seems
to be better to use only one type in the text for consistency
(either only "failure" or only "error") and to avoid possible
confusion.