A common misconception is that a connecting mailserver's IP address should resolve back to that mailserver's hostname. That is not the case, and it's usually not even possible when you have no control over your PTR records.
1) the mailserver's IP address should resolve to a valid hostname (which may also be dsl.ip.your.isp.com)
2) that hostname should resolve back to the same IP address.
If both conditions are not satisfied, some anti-spam systems may flag your mail as 'suspect' or even 'spam', or add some SpamAssassin points to the spam score.
If you have a public IP address from an ISP, and it does not resolve correctly both ways, your ISP should really fix that.