Try using another mail server is the best thing that I can suggest. Either that, or check into looking to see if the mail server that is doing the auto-responding is not on any blacklists. If they are, then your Host will have to work on getting their act cleaned up.
There are slews of lists out there, and I am not sure which one Yahoo is pulling from. They do not really tell you that in their FAQ's that I could find. One of them (pretty sure) is CBL which is a list of servers with Open Relays. It is easy to get removed from the list, but you will wind right back on it if you do not correct the problem.
Do you have a copy of one of these confirmation mails, including the header information? If you do, there are some things you can do to test the mail services at Yahoo and Hotmail.
First you want to figure out if it is getting bounced because of the content in the mail, or if it is the IP address of the originating server. If you cut and paste the entire message into some mail, include the same subject line, then mail it from a different mail account that is NOT the same as your auto-responder account (use a friends account for instance). Then see if the mail is still getting bounced in the same fashion. If so, then it is something in your wording that is triggering some kind of spam filter possibly.
The header information will give you the IP so you can run a blacklist check on it. You can use
http://www.dnsstuff.com/ and they have a spam database lookup you can use. It is located at the top of the middle column.
Confirmation Email Compatability
Linear Mode
