Hello,
What is your business domain name?
Does your outgoing mail server belong to your company or does it belong to "the guy"? What is the IP address of that mail server?
Is that mail server dedicated to your company, or do other companies also send email using that server?
Most likely that mail server is experiencing a new abuse problem such as a compromised account being used for spamming.
Please ask "the guy" for a copy of the mail logs related to your missing email that was sent to Hotmail.
If you know how, please post the full headers of an email that successfully made it to Gmail, or if you don't want those in public please send them to me in a private message.