As Dom said, you don't need an MX record if it's an outbound-only mail server, the MX record is only needed for inbound mail.
There is no such thing as an "A-record for an IP-address". Domains can have A-records, IP-addresses can only have PTR-records (Reverse-DNS). Most mail services only require you to have some PTR-record for your IP that does not look like a dynamic record (e.g. NOT 12-34-56-78.reverse.domain.com).
Instead, it should point to some domain name resolving to the server's IP. Some really strict mail providers will also check that your mail server's HELO corresponds with your PTR record, so it's a good idea that they're the same.
The whole idea behind the MX record is to specify a host or hosts which can accept mail for a domain. As specified in RFC 1035, the MX record contains a domain name. It must therefore point to a host which itself can be resolved in the DNS. An IP address could not be used as it would be interpreted as an unqualified domain name, which cannot be resolved.
The reasons for this in the 1980s, when the specs were originally written, are almost the same as the reasons for it today: A host may be connected to multiple networks and use multiple protocols.
Back in the 80s, it was not uncommon to have mail gateways which connected both to the (relatively new) Internet which used TCP/IP and to other legacy networks, which often used other protocols. Specifying MX in this way allowed for DNS records which could identify how to reach such a host on a network other than the Internet, such as Chaosnet. In practice, though, this almost never happened; virtually everyone re-engineered their networks to become part of the Internet instead.
Today, the situation is that a host may be reached by multiple protocols (IPv4 and IPv6) and by multiple IP addresses in each protocol. A single MX record can't possibly list more than one address, so the only option is to point to a host, where all of that host's addresses can then be looked up. (As a performance optimization, the DNS server will send along the address records for the host in the response additional section if it has authoritative records for them, saving a round trip.)
There is also the situation that arises when your mail exchangers are provided by a third party (e.g. Google Apps or Office 365). You point your MX records to their hostnames, but it may occur that the service provider needs to change the mail servers' IP addresses. Since you have pointed to a host, the service provider can do this transparently and you don't have to make any changes to your records.
Best Answer
While technically you can point an MX record to an IP many places won't let you so just create an A record called mail and point it to your IP for the domain and then point the MX record to that.