Possibly wrong headers?
I've not experienced any problems with this, however during a deep dive into the mailing system recently with @herbdool it appears that not all Civi's compose→send codepaths support adding headers in alterMailParams through the $params['headers']['X-Some-Header']
method. This is what this extension uses to add necessary headers. It might be safer to set $params['X-Some-Header']
instead or as well, since it looks like headers set this way take precedence.
Side note, I believe SMTP header names are case insensitive, but our handling could easily allow $params['headers']['X-Some-header']
and $params['X-Some-Header']
to accidentally generate two headers.