Follow these steps:
- Check the SMTP Log under the Logs section in Server app. Proceed to step 2 if you see a line containing this message:
"FAILED: Can't call method "isa" on unblessed reference at /Applications/Server.app/Contents/ServerRoot/usr/bin/amavisd line 6062. (in reply to end of DATA command))"
- In Server app, select the Mail service and click the Edit Filters Settings button. Change the minimum score for junk mail by moving the slider in either direction, then click the OK button to save the changes. Mail clients should now be able to receive mail.