Do you know WHY your emails are being filtered? It could be anything from your server being on a blacklist to just one single word being on the filter list. More details are needed.
Well, why we are into this, AOL is always harder to get past then hotmail. it is either your host. or.... it is the way your title is.
The bottom line is there are dozens of reasons why an email could be blocked or bounced. Like I said, your server could be blacklisted (usually the case with AOhell), certain words in your email raise flags, your email contains links, your email is HTML-based, or your email is encoded using a format the receiving server doesn't like (like base64). These are the more common reasons for emails not getting thru filters.
I don't think that my server is in a backlist, because i have an ecard service in my site and when i send an ecard it goes go to the inbox but if I send and invitation to my friend it goes to the junk mail, i tried using the same code of the ecards for the invitations but it didn't work.
If you're using the same code and server, then it has to be something in the content itself, a specific word (or words) or link. Sometimes legit words or phrases raise flags; I manage a mailing list for a charity that raises money for kids, unfortunately they're called "Young Variety - The Children's Charity". It seems the words "young" and "children" are preventing some emails from getting thru to the recipients. Of course, it could also be "charity" even, I can't know since there isn't a public, definitive list of banned/restricted words. Also, is the "From" or "reply-to" different between the ecard and invite? If it is it could be as simple as your recipient not having the correct address in their address book.
Trying to get emails from spam folders to the inbox is just like trying to get websites to the top of the search engines. Once somebody figures it out they could make a fortune. There is no specific answer.
Hotmail probably allows people to report when something should not have been marked as spam. Your friend could do this and/or place you on his whitelist, if hotmail allows such a thing. Since ecards sent from your server get through using identical programs and routings, you know that your email server and/or routing is not the problem. That means the problem is with the content of the message. Reword the "inivitation" so it sounds less spammy. Make sure you do not include made up words and junk intended to confuse filters. If you are including links make sure they contain no obfuscation techniques and check any domains in the messages against RBLs to make sure they are not banned.