Scratching my head with this problem.
Email notifications are not getting generated from the inside network.
Inside -> FWSM -> 6500 (NAT) -> 2nd Level ASA -> 1st Level ASA (PAT)
The SMTP access is allowed throughout. I can see Build/Teardown on FWSM and 2nd Level ASA. However, on 1st Level ASA I can see 'Deny TCP (no connection)..RST Flag' in the logs of 1st Level ASA for the return traffic.
Going through forums etc, I believe there are mainly two reasons for this error 1) Asymmetric routing 2) SMTP inspection
In my case, neither Asymmetric routing nor SMTP inspection is occuring. Still I get the above error.