Setting the MX record in the public-facing DNS is not enough. It'll sort-of work, and you'll likely get some mail. Other mail will be silently dropped, and the numbers of other mail servers that will silently drop mail intended for or received from your mail server is likely to increase over time.
And you have a vastly larger issue here, if you can't use Server Admin, and (based on this thread) it appears that you've reinstalled your own version of Apache on this server.
Proper DNS is fundamental to the operation of servers. DNS is also fundamental to the operation of network security.
Again, your forward DNS (name to address), reverse DNS (address to name) and the MX all have to match for the primary name of the server, or mail to and from the server will be dropped by other mail servers. This is because the SMTP protocol requires the mail server to have a DNS A record - also called a DNS machine record, or DNS host record - as its primary name, and yours does not. Yours has an alias record, also called a CNAME.
Hosts with CNAMEs, as well as hosts with reverse DNS names based on DSL or other dynamic names, and also hosts with dynamic addresses where the ISP has added those to various of the filtering databases, are considered spam engines.
If your DNS is not configured, you'll need to use an SMTP relay. Or you'll have mail dropped.
Here is an introduction to setting up DNS on OS X Server, and there's a link from there to an article on setting up your external DNS. That uses Server Admin, though.
Given you're avoiding the Server Admin tool, you may well end up need to manage your DNS files manually as well. (Once you start "off-roading" with an OS X Server configuration, you're almost better off migrating to a Unix or Linux server. To manage these components without either the Server Admin GUI or the serveradmin tool at the command line, you'll need to edit the various configuration files for the services directly.)
Once your internal and external DNS are configured and confirmed, you'll want to use IMAP and not POP. POP is a more limited protocol.
To use aliases for recipients, the OS X Server documentation (previously linked) provides two means of configuring users across different virtual hosts within the postfix configuration; aliases and related. One is more flexable, but is manually managed, while the other uses Server Admin.
In general with this configuration, I'd suggest routing your mail through another mail server as a way to avoid the overhead of managing the mail server, avoiding the DNS set-up, and given you're avoiding Server Admin. The web sites I manage get attacked multiple times a day, as do the mail servers, and many of those folks are looking to use vulnerabilities in the web sites and the mail servers to send out spam, and the attacks are continuously evolving and improving. And you're working with a substantially more complex configuration than the documentation covers.
I'd suggest looking around for a way to add just the mod_wsgi module without rebuilding and reloading Apache; there are some postings around with some details; here's a very high level overview of the sequence. (Here's an older Leopard discussion of this module. There are probably better descriptions around; it's been a while since I've needed to side-build an Apache module, given that the php versions and libgd/gd are mostly-sorted out on Snow Leopard an Lion.)
And in all seriousness, a migration to Linux or a BSD is probably a better fit here, particularly if you're planning to rebuild hunks of the platform, and thus end up needing to avoiding using the integrated management tools. The way you're managing this server is certainly a fine and viable approach, and it's the exact target market for managing Linux and BSD platforms; full customization. It's just not where OS X Server is aimed.
