Check three things:
1. Make certain the SENDER is SMTP AUTHENTICATED
2. If the sender if FORWARDING to another e-mail address, make certain you have SRS enabled in SmarterMail. If it's not enabled, and the recipient is forwarding, it will break SPF and you will be rejected.
3. Make certain the SENDER is using the same SENT FROM and REPLY TO e-mail addresses, IE: not sending on one account and expecting a response on another - now blocked by many large ISPs.
Phonr: (773) 491-9019
Phone: (224) 444-0169
E-Mail and DNS Security Specialist
Network Security Specialist
Customer Service Portal: https://portal.chicagonettech.com
Security Blog: http://networkbastion.blogspot.com/
Web and E-Mail Hosting, E-Mail Security and Consulting