SMTP answer message without ACK bit ???

[SuMic]

Hi list,

on our firewall we have the following rules:
Rules are applied for traffic from DMZ -> Internet

Rule 1
!--- Our email-server tries to establish a SMTP
!--- (email) connection to an Internet-Server
permit tcp <ip our email server> gt 1023 any eq smtp

Rule 2
!--- answer from a SMTP (email) connection which a host
!--- from Internet has established to our Email-server

a) permit tcp <ip our email server> eq smtp any gt 1023
b) permit tcp <ip our email server> eq smtp any gt 1023 ack



Now the question:
with a) all works fine
with b)if i enable in rule 2: check if ACK (acknowledge) bit is set, only some emails from few different email-servers will be blocked?

e.g.:
<ip our email server> 25 <192> <highport> 195.20.224.149
is blocked by rule 2b)

Any suggestions???