Duplicate MIME-Version header

[johnjamesjacoby]

From ​#WP22492:

In certain cases, Wordpress sends emails containing two "MIME-Version" headers. This causes email rejections from some Amazon SES SMTP servers in particular.

To reliably reproduce, a default multisite install with BuddyPress can be used. When a user tries to register and create a website, the activation email contains this bug. Unfortunately I don't know how to reproduce this error with Wordpress itself, but I believe this bug is related to the Wordpress core. If any plugin happens to set a custom header of "MIME-Version" to an email message, Wordpress core is the culprit for the insertion of the duplicate mail header.

[r-a-y]

Confirmed.

For some reason, we are setting the email headers and the 'From' email address and name in the activation emails for multisite:
​https://buddypress.trac.wordpress.org/browser/tags/1.8.1/bp-core/bp-core-filters.php#L257

For single site, we do not override the 'From' email address or the 'From' name. See the bp_core_signup_send_validation_email() function.

I propose we remove the email headers for multisite activation emails, unless there was a good reason to set the 'From' name in the first place.

[boonebgorges]

It looks like the MIME-Version header was added in ​http://buddypress.trac.wordpress.org/changeset/2832/. Maybe there was a reason for doing this when WPMU was handling outgoing mail (though I'm not sure what it would have been), but in any case, I can't think of any good reason for doing so now. So I think that removing it is fine.

Should the same thing happen in bp_core_activation_signup_user_notification()?

[r-a-y]

In 7463:

Remove unnecessary email headers in multisite activation emails.

BuddyPress does not set email headers for single-site activation emails,
so the same thing should occur for multisite.

Fixes issue with setting the 'MIME-Version' header twice, which could
result in the activation email being rejected in some cases.

Fixes #5186.

Props ryderlewis. See ​#WP22492.