Question about Outlook Express

I am having a strange problem and I can't figure what the cause of it is. Ever since I switched from a Gmail account to a Live.com email account (at the same time I switched to a Windows 8 computer and a Windows Phone 8 smartphone), I have been having email threading issues.

On my end, the conversations work perfectly fine, each email in a conversation is grouped together and very organized. However, my friend and colleagues that use Gmail keep telling me that every time I reply to an email of theirs it creates a whole new thread rather than having them all grouped together. The common denominator here is my Live.com account as none of the other people they correspond with use Microsoft email and I am the only contact that causes this problem.

I am wondering if this is a problem with the Mail apps on Windows 8 and Windows Phone 8 or a glitch in Microsoft's email system. I would really, really like to know how to fix it as it is a recurring problem.

Thanks so much! Hope someone can help me.