Posted by Gordon Burditt on 06/09/05 18:59

>I can't speak for Yahoo for I've never used their mail, but some hosts just
>reject all mail from servers that do not meet certain (often secret)
>conditions:
>
>* IP is not blacklisted.
>* Reverse DNS lookup does not resolve to hostname.
>* Messages do not include certain words.
>* Attachment are not of given type (I know servers that reject ZIP files).

There are more conditions than this to worry about. For example,
mail may be rejected :

- The server identified by the From: address (MX records on the domain)
is localhost, 127.0.0.1, a private address, or doesn't exist.
- The server identified by the From: address does not accept any bounce
messages. ( MAIL FROM:<> to the SMTP server always fails, either
immediately or after RCPT TO:).
- The server identified by the From: address will not accept the address
in the From: line as a recipient (Exim callout verify).
- The mail contains an attachment (ANY attachment). This is fairly
unusual, though.
- The mail contains an attachment of apparent executable type
(exe, com, pif, scr).
- The domain identified by the From: address publishes SPF records and
the mail sender isn't listed.
- The domain identified by the From: address is on a list of known
or suspected banks (especially banks being phished) and it fails
to publish SPF records.
- Any domain in the headers (To:, Cc:, Reply-to:, From:) ends in a
period, contains two consecutive periods, or is "<Undisclosed Recipients:;>",
even if the offending domain is not a sender or recipient of this
copy of the message.

Gordon L. Burditt

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