How do you find a IP address for a SmtpServer

Where or how do you find a IP address for a SmtpServer when you know the smtp server: "smtp.email.com"?

$PlainPassword = "mainpassword"
$SecurePassword = $PlainPassword | ConvertTo-SecureString -AsPlainText -Force
$Username = "MYSITE\Administrator"
$Credentials = New-Object System.Management.Automation.PSCredential -ArgumentList $Username, $SecurePassword
$mailprops=@{
    Subject = 'This Subject'
    Body = 'The Body and the subBody'
    To = 'emailto@mail.com'
    From = 'emailfrom@email.com'
    SmtpServer = '???'
}
Send-MailMessage $mailprops -Credential $Credentials

Error Code:

Unable to connect to remote server.