Get all mailboxes from one company in large hybrid environment

Hi,

I am having issue with a "simple" task. I have to pull out all mailboxes that concern only my company from our large hybrid (exchange/office365) environment where all other companies reside (we are talking in tens of thousands). I tried over console, did not succeed, with powershell I tried for start:

get-mailbox  -resultsize unlimited | where {$_.displayname -like "xy*"}, I also tried PrimarySmtpAddress with *@companyname.com - no dice.

Please help,

Cheers,