Dynamic:BlakJak: 1) Use DNS and direct-to-MX delivery.We have had good results with DNS and direct to MX delivery (providing PTR records are set up) . Occasionally we have had an ISP-assigned Static IP addresses that are in a range configured as dynamic/dialup and have had to be on at the ISP multiple times for this to be sorted.
- Required outbound Port 25 access
- Introduced potentially problematic delivery due to the issue flagged above around IP address reputation - coming from a broadband customer pool is going to see people judge you as such (general rule is that residential-grade IP addresses shouldn't be doing SMTP - and sorry if you run a business on ADSL, but that's what it is...
To be clear it's not simply the existence of a PTR record, it's a matching PTR-and-A (or 'forward-and-reverse') combination.
But these are only part of the picture.
For many years mail service operators have immediately 'marked down' email coming from netblocks known to host end-users, in large part because such a substantial amount of spam comes from compromised machines on the back of end-user netblocks.
It's not black and white; you may well have had lots of success but I would not want to assume this will continue. On the other hand should you experience problems there's also a chancel you can negotiate with the remote party about how they treat your IP. There's also a chance they'll be a large player who won't care, or an RBL operator (SORBS come to mind) who also, won't care.
In general ISP mail servers are safer - so long as your ISP has a good rep for dealing with compromised machines when they come up, to preserve the rep of _their_ IP addresses - than doing direct SMTP delivery from an IP in an xDSL netblock. However you may be lucky enough to not be in a netblock so-categorised or not correspond with people who use those sorts of services.