How to switch to another domain and get-aduser

get-aduser -Server "servername" -Identity %username% -Properties *

get-aduser -Server "testdomain.test.net" -Identity testuser -Properties *

These work when you have the username. Also less to type than using the -filter property.

EDIT: Formatting.

Try specifying a DC in DomainB using the -Server property. Ex:

Get-ADUser -Server "dc01.DomainB.local" -Filter {EmailAddress -like "*Smith_Karla*"} -Properties EmailAddress

I just want to add that if you don't inheritently know the name of a domain controller, you can get the closest one, pass it's hostname to the -Server argument.

$dc = Get-ADDomainController -DomainName example.com -Discover -NextClosestSite

Get-ADUser -Server $dc.HostName[0] `
    -Filter { EmailAddress -Like "*Smith_Karla*" } `
    -Properties EmailAddress