Alias             : User01
EmailAddresses_1  : X500:/o=bla bla bla b
EmailAddresses_2  : x500:/o=bla1 bla1 bla1 bla1
EmailAddresses_3  : smtp:USR1@testdomain1.com
EmailAddresses_4  : smtp:user01@testdomain1.com
EmailAddresses_5  : smtp:user1@testdomain2.com
EmailAddresses_6  : SMTP:user001@testdomain1.com
EmailAddresses_7  : SIP:usr01@testdomain1.com
EmailAddresses_8  : smtp:u1@testdomain2.com
EmailAddresses_9  :
EmailAddresses_10 :

$Test = [PSCustomObject]@{
    Alias            = 'User01'
    EmailAddresses_1 = 'X500:/o=bla bla bla b'
    EmailAddresses_2 = 'x500:/o=bla1 bla1 bla1 bla1'
    EmailAddresses_3 = 'smtp:USR1@testdomain1.com'
    EmailAddresses_4 = 'smtp:user01@testdomain1.com'
    EmailAddresses_5 = 'smtp:user1@testdomain2.com'
    EmailAddresses_6  = 'SMTP:user001@testdomain1.com'
    EmailAddresses_7  = 'SIP:usr01@testdomain1.com'
    EmailAddresses_8  = 'smtp:u1@testdomain2.com'
    EmailAddresses_9  = $null
    EmailAddresses_10 = $null
}

$SearchString = 'SIP:'

$Found = Foreach ($T in $Test) {
    $Properties = $Test | Get-Member | Where {($_.MemberType -EQ 'NoteProperty') -and ($_.Name -like 'EmailAddresses*')}
    Foreach ($P in $Properties) {
        if ($T.($P.Name) -like "$SearchString*") {
            $T.Alias
        }
    }
}

$Found | Select -Unique