[Sender Auth Failure]

[#1]

When I send an email and receive the reply, in the subject is added [Sender Auth Failure]  . I am not sure if added by the receiver - to whom I am sending the email or by PN upon receipt. Can anyone explain?

[#2]

Is this behaviour specific to replies received from one particular correspondent or all of them?

That prefix matches one added by a Symantec Gateway product when doing SPF checks. Does your correspondent's email provider use this product?

[#3]

Not all recipients. It is a large company so difficult to find out what they do and check.I agree with what you say. This implies that PN is not adding the SPF?

[#4]

I'm fairly sure this subject-line tag is not being added by Plusnet. If they were doing this I would expect to see other mentions here. Also a message that would have failed SPF validation came through unmarked to me this morning.

When you send your message to the company are you using your Plusnet address and using relay.plus.net as the outgoing mail server? Since Plusnet don't publish an SPF policy I'm wondering what the company could be checking to cause the failure tag to be added.

It would be peculiar if the company were checking and tagging their own outgoing messages.

[#5]

It's not us. As mentioned by @spraxyt, it looks to have been added by a Symantec product.