What does the massage "The email you sent was rejected because there was a validation error" mean?

This is a mail2web issue, not mailhandler's.
I guess it means that the email address you use to reply to a mail, must be the same as the email address the mail was sent to.

mail2web adds a special string to email notifications, and also changes the reply-to address on mails from the site. You must reply to that exact address.

Fixing the title for findability

This means that the security signature that mail2web put in the email you received from mail2web was not found when you replied back to the email. Are you still having this problem? If so, please provide more details like what exactly you are doing, the email client you are using, etc. Thanks.

I too am receiving this error message. I am following the documenation http://drupal.org/node/542118 and am on Optional Step 5 testing. I have sent test message from 2 vaild user emails on the drupal site. 1 @yahoo.com and 1 @gmail.com. Also changed admin email under my account of admin to work email @edc.org I have not typed anything in the mailbox configuration entry "from header".

using mailhandler 6x-1.8

To clarify. I am not replying to any message. I am trying to post a new message. From my admin user email address.

@wheels394

I just spent the better part of a day messing with mail2web. It is not for posting new content on a drupal site. It is for posting comments on site content or replying to another comment via email notifications received about new content or new comments.

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What does this mean: "The email you sent to events@mydomain.com was rejected because there was a validation error."

Exactly what it says.
- Let's say uid1 posts content "the sky is falling".
- uid2 posts a comment to "the sky is falling" of "what are you Chicken Little?"
- uid1 gets a notification of new comment by uid2 and instead of having to log into the site to reply, uid1 replies to the email
- but uid1 screws up or his email app screws up and erases the validation (message id at bottom of email) or has emails forwarded to another address and attempts to reply from there.... mail2web needs to see the validation id and mail2web only let's in emails from registered users to block spam... if either one of these 2 things are missing mail2web spits back the validation error.
- Since the module is still beta and email apps having a ton of different ways of replying to stuff this module as is won't work for every user.

This is correct, mail2web is for replying to notifications generated by messaging/notifications when new nodes are posted. When you reply to the notification, your reply will be posted as a comment. Mail2web is not for posting new nodes. However, you can still post new nodes. What you need to do is go to /admin/messaging/mail2web and where it says "Passthru to mailhandler module:" set this to "On". If any mail is sent to Drupal using the mail2web mailbox, and it has no headers, it will not throw an error w/ this enabled, but instead pass the mail to mailhandler for processing.

@wheels394 please close this if it answers your question. I am going to tag this post FAQ and will add this question to a FAQ I will develop. Thanks

Thanks it helped me !

Switch to mailcomment project. Mail2web is now known as "mailcomment" due to a trademark issue with the name mail2web.

I was getting this message as well and the solution was in #641758: Messageid backup for subdomains, and more robust checking in general, and whitespace cleanup..