Duplicate MIME-Version header
|Reported by:||ryderlewis||Owned by:|
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.
Patch attached to this report, it's a trivial fix. All the fix does is checks to see if "MIME-Version" was already included in custom mail headers, and if so, don't insert the duplicate header.