To resolve this, you need to Enable SRS to REWRITE HEADERS with updated SPF information when forwarding messages.
This will ensure that the SPF is picked up for the domain and an additional HEADER line is inserted to enable the SRS
"SRS - To enable or disable SRS (the ability for the mail server to re-write the senders email address so that forwarded messages pass SPF checks) for mail, select the appropriate option from the list."
To do this, login as the SmarterMail ADMIN and go to SECURITY ===> ANTISPAM ====> OPTIONS and ENABLE SRS
For more information on SRS header rewrites, see the section on SRS in the OPEN SPF website, at:
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