Sadly, this is for me the only dark point being a “Yunohoster”.
If you make a forum research using the keyword “Spam”, you’ll see that this problem affects us all…
I did start a topic on this subject with some details if you can understand French
Big email providers don’t like small personal servers (which is the true spirit of email by the way) and don’t respect the international email standards. It’s a fact, we can’t do nothing except try to convince our friends to close their accounts and build their own servers !
There are few things you can do 
- Setup your reverse DNS. That one is very important. I can help you but you need to tell us : are you on a VPS ? Computer at home ?
- Send an email to all your friends and check who has received it one by one, ask them to reply to you, un-spam your message, white-flag you etc…
- Register to the Microsoft IP lookup (will help you if any troubles with Hotmail) Smart Network Data Services
(just for the fun, Microsoft will ask you to register for a microsoft email address…) - WAIT, WAIT, WAIT…
@kanhu is right, you’ll have to be (very) patient.
With time, and good friends white flagging your mail address, un-spamming your messages, replying to you, you’ll see your reliability rate increase slowly. I still have few problems but nothing compared to the beginning.
Needless to say it’s very frustrating and causes some bad surprises with mails lost or never seen by receiver, etc… Bon courage !