How to read an MX record
The preference number sets the order, not the importance: a sender tries the host with the lowest number first and moves on to the next one only when it cannot be reached. When two hosts share the same number, the load is split between them.
An MX record has to point at a host name that has an A or AAAA record. Pointing it at a CNAME goes against RFC 2181, and part of the mail servers on the internet reject such a domain instead of guessing what was meant.
PTR records and delivery
If your own server sends mail, the address it sends from needs a PTR record pointing back to the host name of the server, and that host name should resolve to the same address. Only the holder of the IP address can create a PTR record, so it is requested from the hosting company or the data centre.