For on premise exchange servers:
Launch an Exchange Management Shell and run this
command, substituting domain.com for the you want to look for such as mydomain .com
Get-Mailbox -resultsize unlimited | Select-Object displayname -expandproperty
emailaddresses| Where-Object {$_.smtpaddress -like "*domain.com*"}|
Select-Object displayName,SmtpAddress | Export-csv C:\domain.csv
This will save a .csv file to your C drive with a list of accounts that have
an email address for that domain
For Exchange Online:
Launch a powershell session and run Connect-msolservice. Enter your 365 credentials then run the command below
Get-MsolUser -ReturnDeletedUsers | Where {$_.ProxyAddresses -like "*@mydomain.com"} | FL DisplayName,UserPrincipalName , ProxyAddresses
This will output the results to your powershell terminal
