I noticed the failed Travis build, but the failing tests seem unrelated to the changes in this PR. I'm not sure how I can address that. Can anyone advise please?

Thank you @cvmiert.

You wrote in the introductory comment:
"The Return-Path is usually configured in addition to the Sender and/or From header: it allows for email bounces and complaints to be sent to a dedicated email address. It should therefor not be used as the first candidate header to determine the sender."

You are correct that the "return-path" is set in addition to the "sender" and/or "from". And it is used for bounces of emails. You are also right that "return-path" should not be used as the first header candidate to determine the sender.

But please don't use "return-path" as the last candidate. The last one must be "from". "from" is alway set and thus is the last candidate.
If you want to use bounces, set the "return-path". And in this case, the sender must be the "return-path" email, not the "from" email. And only if the "sender" header is set, it will override the "return- path".
I am not sure if this is correct for all email daemons, but for sendmail it is necessary.