Hi,
I think I understand your post but not quite 100% sure of your topology , pls correct if my understanding is wrong, also look at asdm real time log to give more clues what the problem could be.
Where is your webmail server, on the outside or inside ? could u provide more info on where wemail server is located.
LAN-ASA5510Outside-SW-InternetRT, is this your topology ?
if so assume example:
ASA-Outside IP = 99.99.99.99
Outside router Ethernet interface 99.99.99.100
Outside switch IP = 99.99.99.101 its DG 100
LAN Network 192.168.1.0/24
LAN segment needs to be PATed to reach oustide router and outside switch external IP addresses.
This is all you need in ASA-5510 to reach external switch and/or router from any 192.168.1.0/24 host.
global (outside ) 1 interface
nat (inside) 1 192.168.1.0 255.255.255.0
or
global (outside ) 1 interface
nat (inside) 1 0.0.0.0 0.0.0.0
Regards
Jorge Rodriguez