The easiest way to determine the issue would be to set up packet captures on the ASA :

Configure Packet Capture on ASA :

Wan Inbound Traffic :

access-list CAPOUT extended permit tcp object-group Messagelabs any
access-list CAPOUT line 2 extended deny tcp any any eq 25


!
!

Lan Inbound Traffic :


access-list CAPIN extended permit tcp host 172.16.0.125 any eq 25
access-list CAPIN line 2 extended deny tcp any any eq 25

!
!

Bind to Interface :

cap CAPIN access-list CAPIN interface inside
cap CAPOUT access-list CAPOUT interface outside

You can check the captures with :

show cap CAPIN
show cap CAPOUT
!

Packet Capture :

show capture CAPIN detail packet-number 5 dump
show capture CAPOUT detail packet-number 5 dump
!

Display <number> of packets in capture :

show capture CAPOUT count 10