I need to open up port 587 on my firewall for a custom program that sends out e-mails to a few people outside the organization. However, it seems that the destination IP address that the vendor that made the program insists is correct does not work when I set up the firewall rule. If I open up full port 587 outbound access though, the e-mails goes through fine.
If I wanted to figure out the correct destination IP address, would I be able to do this on my own? I know the domain that the e-mails are sent to, and when looking up this info, it seems that they may have mail hosted by Rackspace. Does this mean that I should add the Rackspace MX record IPs as my destination address?
What I would probably do is enable logging for that port on your firewall. Send out an email and see what it connects to. Then compare it to what you have been told by the vendor.