I have forwarders set up to send mail off the server, and the domain is using SPF records. Mail being forwarded is failing to be received by the recipient's server because it's failing SPF checks. Error messages vary, but look generally like this:
SMTP error from remote mail server after DATA: 550 #5.7.1 SPF unauthorized mail is prohibited.
What can I do so that this mail is delivered?
You can enable SRS in the Exim Configuration Manager's Basic Editor. Read more about this option in our documentation for the Basic Editor.