Hey so i have a 2013 exchange server that has 2 domains. My default domain is main.com and my secondary is main2.com. I've double checked and the default is still main.com When i add a new user it defaults to main2.com and I can't figure out how to change that.

go to ecp -> select the user -> click edit -> select email address -> select the email you want primary -> click save

Thanks,

Unless i'm blind there's no way to select the email i want as primary. It shows my support@main2.com in bold and support@main.com as not. There's no buttons and I can't delete the main2.com from the list.

select support@main2.com    " you can see - mark after pencil pic , - means remove. just point your cursor on - pic , it will show the message remove

It's greyed out 

remove the tic mark of Automatically update email addresses based on the email address policy applied to this recipient and then try again

Sorry to say it's still greyed out.

from my side it is working, now this one is tweak to another issue. i think it may be privilege issue

When i edit the support@main2.com and change the domain to main.com and save it I get this error

Couldn't update the primary SMTP address because this mailbox is configured to use an email address policy. To disable the email address policy for this mailbox, run the command with the EmailAddressPolicyEnabled parameter set to $false.

problem solved????

Nope. I've tried that command before but do I need to set it back to true after?