All mail that gets sent to my @mcmaster.ca goes to @gmail.com. When I log in my @mcmaster.ca account through sso, there are no emails. How do I unlink the accounts, I've already tried googling but nothing works. I need them split up since whenever I get email sent to my @mcmaster.ca, my reply ends up from my @gmail.com account. Since nothing gets sent to my @mcmaster.ca inbox, I can't reply using @mcmaster.ca.

Click on the little gear in the upper right corner, then click settings, then click "forwarding and POP/IMAP".

Your POP is probably enabled, which is fine, just make sure the drop-down menu in that section is set to "keep McMaster University mail's copy in the inbox." That way, it won't automatically delete the messages from your inbox.

Alternatively (or additionally), you could set up your gmail to be able to respond as if you were responding from your Mac account. To do this, click on the gear in the upper right corner, then click settings, then click accounts.

The second section there should be "send mail as". You just have to click "add another email address you own" and follow the steps from there.

I recommend disabling forwarding altogether. Then use IMAP with outlook/thunderbird or a smartphone to combine your email from both accounts.