This feature is generally referred to as "SRS" or the Sender Rewriting Scheme, which is used to deliver to mail hosts that experience delivery because of SPF senders.

What is SPF?

A quick explanation of SPF is that it is a simple type of sender authentication that verifies whether the server sending the message is authorized to send mail with the message's given Return-Path address (also known as the Envelope Sender address).

Why is a workaround needed?

Many mail hosts will block or delay messages that fail an SPF check. When using our service, all your mail should come from our servers. Your mail host will check the SPF record for the message sender and see that our servers are not included in the SPF-authorized server list for the sender. So your mail host will think the SPF for the message failed even if the SPF passed when we received it from the original sending server.

What does the workaround do?

The SPF Workaround Delivery feature replaces the "return-path" email address of the message with an email address that will always pass SPF checks coming from a generic domain owned by SpamHero. If the recipient of the message responds to the return-path email address instead of the one listed on the From line, SpamHero forwards the message back to the original sender.

See also: Resolving delivery issues with SPF senders