Domain Cancelling

Hello, I'm the "technical guy" for a small (very small) ISP sort of subisiduary of a larger ISP.
I just recently (2 weeks ago) transferred a domain name with all it's services (email, web hosting, etc...) over to a new provider.
The system I use, and used to cancel this domain is a UNIX server.
The issue is that on certain computers, when trying to email the customer, I get a bounce back indicating that at least my computer is still trying to resolve to the old server. The response is below:

----- The following addresses had permanent fatal errors -----
<advertising@domain.ca>
(reason: 550 5.1.1 <advertising@domain.ca>... User unknown)

----- Transcript of session follows -----
... while talking to relay1.mydomain.net.:
>>> DATA
<<< 550 5.1.1 <advertising@domain.ca>... User unknown
550 5.1.1 <advertising@domain.ca>... User unknown
<<< 503 5.0.0 Need RCPT (recipient)

On my end, i'm 100% i've disabled any trace of DNS and emails on my server. On other computers, the emails are resolved just fine to it's new host.

I originally thought that it was a resolving issue that would be fine in a week, but it's been over that now and this is still occurring.

Did I do something wrong on my end? I could elaborate more on the steps I take when cancelling a domain. Does it need more time to resolve still? I even did a DNS flush on my computer if that helps.

Any advice would be greatly appreciated!