I am having problems sending mail from postfix on a dsl connection, to hotmail and yahoo

My configuration has not changed for at least two months, my IP is not blacklisted anywhere, but hotmail's problems started about month ago and yahoo was still working two weeks ago.

I get the following error message:

(host mx1.mail.yahoo.com[64.156.215.8] said: 501 Syntax error in parameters or arguments (in reply to MAIL FROM command))

(host mx1.hotmail.com[65.54.166.99] said: 501 Invalid Address (in reply to MAIL FROM command))

I can think of two posible problems:
1) I do not have the reverse DNS set up (and was not planning on doing so, just to try to reduce the spam)
2) yahoo and hotmail are blocking mail from dsl and dial up connections.

Can anyone shed some light onto this?

TIA

I'm running a mail server at my house on a DSL connection and it handles about 5k messages per day and I don't think there is anyone blocking me at all... much less for being on a DSL connection. I know there are some ISPs who block mail from dynamic IP pools, so you're right.. they are doing it... but none of the ones I interact with are preventing me from sending mail, and certainly not Hotmail or Yahoo... I have a lot of users on those services and they haven't experienced any problems.

I don't know anything about Postfix, but I wouldn't think MAIL FROM had anything to do with reverse DNS. I know qmail has the ability to block messages coming from hosts that don't have reverse DNS setup, so if they were going to block you for not having reverse DNS, the error should be more clear about that.

In my experience, Hotmail and Yahoo are the most easy-going when it comes to accepting mail... the amount of spam I get at my hotmail account is a testament to that! So I would venture to say that somethign is very wrong with your setup, otherwise they would be accepting your mail.

Sorry, I dont think that helped you much, but those are my 2 bits.

Thanx for the reply,

It did help in a way, I know I need to look further

Strange thing is that I could send mail to them for wel over a month since I made the last change. Another friend had a similar problem, If yahoo and hotmail have blocked for eiter reason I suppose this would be a good thing. I only use my hotmail account if disclosing my email would likely result in spam and I hardly ever read it anyway.

I have posted it the same question now in alt.comp.mail.postfix. Where it should probably have gone in the first place