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?