Hi,
For a configuration example for opening up SMTP (TCP port 25) on your ASA, please refer to the document below:
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a00806745b8.shtml
Only thing is in our case, we will need to open port 43 as well in the access-list for internet users to be able to access. We will just need add another line to the ACL "outside_int" and replace "smtp" with "443". Then we should be good to go.
Please note that in the guide, it also gives an access-list on the DMZ interface which is not required unless you want to restrict traffic from DMZ flowing outside. Hope this helps.
All the best!
Regards,
Prapanch