To resolve these issues you'll need a combined approach:
1. Enable SMTP authentication on the domain, and under Settings>Protocols>SMTP IN.
2. Deploy SPF records, DKIM, etc to verify your outbound emails against.
3. Increase the weight associated with failed SPF checks under Settings>Antispam to force rejection of these messages.
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com