Should I use the Reply-To header when sending emails as a service to others?

Question

Suppose we have an application that acts as a middleman, allowing Company A to send reports to their customers.

Company A --> Company B (me)--> Compan

Answer 1

After reading all of this, I might just embed a hyperlink in the email body like this:

To reply to this email, click here <a href="mailto:...">Third.Party@theirdomain.com</a>

Answer 2

I tested dkarp's solution with gmail and it was filtered to spam. Use the Reply-To header instead (or in addition, although gmail apparently doesn't need it). Here's how linkedin does it:

Sender: messages-noreply@bounce.linkedin.com
From: John Doe via LinkedIn <member@linkedin.com>
Reply-To: John Doe <John.Doe@gmail.com>
To: My Name <My.Name@gmail.com>

Once I switched to this format, gmail is no longer filtering my messages as spam.

Answer 3

You may want to consider placing the customer's name in the From header and your address in the Sender header:

From: Company A <joe.bloggs@a.com>
Sender: notifications@b.com

Most mailers will render this as "From notifications@b.com on behalf of Company A", which is accurate. And then a Reply-To of Company A's address won't seem out of sorts.

From RFC 5322:

The "From:" field specifies the author(s) of the message, that is, the mailbox(es) of the person(s) or system(s) responsible for the writing of the message. The "Sender:" field specifies the mailbox of the agent responsible for the actual transmission of the message. For example, if a secretary were to send a message for another person, the mailbox of the secretary would appear in the "Sender:" field and the mailbox of the actual author would appear in the "From:" field.

Answer 4

Here is worked for me:

Subject: SomeSubject
From:Company B (me)
Reply-to:Company A
To:Company A's customers