Email srv problem???

[drinfo]

I just install a ubuntu mail server on ISP config, it work's great but i have trouble getting email from some domains.

Ex (mydomain.com is the domain on my server):

mydomain.com to live.ca It Work!
live.ca to mydomain.com It Work!

mydomain.com to meublesmarchand.qc.ca It Work!
meublesmarchand.qc.ca to mydomain.com Dosn't work

Why i can't receive from that domain? Why some domains work an other domains bug?

I check every setting in main.cf and master.cf, everything looking good! I have a few domain on this server, so i need to fix it fast.

When a lost an email, i don't have any log for ... i called my customer to check and he ping my server as well

HELP ME!!!!