Bulk Add SmtpAddress to users Exchange 2010 - email

Question

We just recently added a domain to our accepted domains and e-mail address policy. While many of our users follow that policy, we have a third domain that some users have as their primary address that does not follow an address policy. I need to take those users and add an SMTP Address in the format of firstname.lastname#domain2.com.
E.g., john.doe#domain3.com (primary) has john.doe#domain1.com as an alias and needs to have john.doe#domain2.com added as an SMTP Address.
I have the following code but am receiving errors:
$Users = Get-Mailbox -ResultSize Unlimited | Where-Object {($_.PrimarySMTPAddress -like "*domain3.com*)}
foreach ($a in $Users) {
$b = Get-User $a.Primary.SMTPAddress
$a.EmailAddresses.Add("$($b.Firstname + "." + $b.Lastname)#domain2.com")
}
$Users |%{Set-Mailbox $_.PrimarySMTPAddress -EmailAddresses $_.EmailAddresses
Errors are as follows:
Cannot process argument transformation on parameter 'Identity'. Cannot convert the "john.doe#domain3.com" value of type "Microsoft.Exchange.Data.SmtpAddress" to type "Microsoft.Exchange.Configuration.Tasks.UserIdParameter".
+ CategoryInfo : InvalidData: (:) [Get-User], ParameterBindin...mationException
+ FullyQualifiedErrorId : ParameterArgumentTransformationError,Get-User
Exception calling "Add" with "1" argument(s): "The address '.#domain2.com' is invalid: ".#domain2.com" isn't a valid SMTP address. The domain name can't contain spaces and it has to have a prefix and a suffix, such as example.com."
At C:\_scripts\SmtpAdd.ps1:4 char:23
+ $a.emailaddresses.Add <<<< ("$($b.Firstname + "." + $b.LastName)#domain2.com")
+ CategoryInfo : NotSpecified: (:) [], MethodInvocationException
+ FullyQualifiedErrorId : DotNetMethodException
Cannot process argument transformation on parameter 'Identity'. Cannot convert the "john.due#domain3.com" value of type "Microsoft.Exchange.Data.SmtpAddress" to type "Microsoft.Exchange.Configuration.Tasks.MailboxIdParameter".
+ CategoryInfo : InvalidData: (:) [Set-Mailbox], ParameterBindin...mationException
+ FullyQualifiedErrorId : ParameterArgumentTransformationError,Set-Mailbox
Thanks

Answer 1

I think the problem starts with this line:
$b = Get-User $a.Primary.SMTPAddress
Get-User fails to return a valid user as $a.Primary.SMTPAddress doesn't return a type that Get-User can use. That then causes EmailAddresses.Add to fail as $b is null. Set-Mailbox fails for the same reason as Get-User
Try this instead (you'll also need to do this on the Set-Mailbox line):
$b = Get-User $a.Primary.SMTPAddress.ToString()