Cannot send email from my Microsoft Outlook 2003 account to any hotmail accounts

Hi

My name is Chamaine Cruz and I run a Wedding Planning Company in Gibraltar (Europe).

I have started experiencing a few issues last week in which I starting receiving emails with the following subject:

"Mail Delivery System".

I can receive emails from any account in the world but it is when I reply back to any clients who uses a Hotmail account that I receive the below:

I use 2 email accounts.  One is - (Removed PII) and the other one is (Removed PII)

Can someone please help me urgently?  I have weddings planned for as early as next week and need to get in contact with my clients.

Thanks

Chamaine