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I see this issue has also been raised elsewhere (in version 1.1.3), but if there was a fix it wasn't articulated.. and the issue persists in version 1.1.6.

My forum has guest viewing turned off and  guest access to the boards set to deny. Now that Email posting is working, I discovered that the sender gets a "Mail-Fail" response. even though the emailed reply posts fine.
Reading the Emailpost.php file and seeing the global $ssi_guest_access; routine, I finally figured out that if guest access is not enabled for the board being replied to via email, a mail-fail message is generated back at the sender.

(Thinking I needed to keep the board permissions for guest set to deny to prevent unauthorized "guest posts" to those boards), I started to mess around with emailpost.php / SSI guest access routine thinking that might be the culprit, but alas, no joy -it's something/somewhere else..

Is there some way to suppress that response back to the sender in this situation? The posting works just fine, it's just sending a false error message.

(The mail-fail response message containing the explicit server path back to the emailpost.php file is a bit disconcerting, but I'll deal with that as a separate thang..)

OK, I believe I found the solution... no need to mess with code.

To enable mail posting and still prevent guests from viewing the Forum:

1 - Disable ->  [Allow Guests to Browse the Forum ].  {This has nothing to do with the email response issue at all.)
2 - Enable guest access in the board permissions  - after all, unregistered users (guests) can't post or email to boards if they aren't registered in the database as a user (emails from a guest go to moderation by default, instead).

(It appears if guests are allowed to browse the forum, then guest access permissions in boards controls guest viewing {allow} or not {deny} for those specific boards.
It also, alas, triggers the emailpost.php to initiate an error back to the sender if an email post is sent to a board that has guest access denied).

Marked "solved"
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[I've edited these posts to refine the issue symptoms (and highlight my initial erroneous assumptions), as well as document the solution (and potential drawback) for the next admin that steps in this mud-puddle.]
(Perhaps disassociating emailpost.php from "guest users", or suppressing that mail-fail response in this 'guest-denied' situation would be a nice conflict resolution in a future release.]