Email activation problems

Ok been meesing around with this for about a week now, and have had no luck.

Problem:
Email Activation not emailing activation codes. However If i sign up and use a email address at the domain i host the board at, everything works fine.

I have mainly used a couple yahoo and hotmail address's and a road runner address.

Now the odd thing is I have a test board at the same domain, works fine no problems sending activation codes to any emails.

I am currently using 2.2.4 as the main forum and the test forum works under both 2.2.4 and the upgraded version 2.2.5.

I have looked at all the setting te are the same and the only thing that is different is the db.
I must be overlooking something but i will be darned if i know what.

Please help
Thanks In Advance

[Alfo]

update,
I was using an email forwarder on the original board. changed it to the address the test board was using. but still no luck.

Please help
ThankYOu