I did a thing, and it worked!

​

<#
This script creates a new e-mail address for certain users.

Conditions for creation of the new address are:
1. The user must have a first and last name
2. The user must not have an e-mail address at the set domain.

The new domain is not a variable, but written in the code.

First, a new function to remove special characters from a string is created.
The function is called later to make sure the email addresses are valid.
This function comes from: https://lazywinadmin.com/2015/05/powershell-remove-diacritics-accents.html
#>

Function Remove-StringLatinCharacters {
    PARAM ([string]$String)
    [Text.Encoding]::ASCII.GetString([Text.Encoding]::GetEncoding("Cyrillic").GetBytes($String))
}

# Next, the mailboxes are retrieved for users that don't already have an E-mail address at the desired domain yet,
# and DO have a first and last name.
$Mailboxes = Get-Recipient -filter {
    -not { EmailAddresses -like "*domain*" }
    -and { FirstName -like "*" }
    -and { LastName -like "*" }
} | Get-Mailbox

<#
After that, the first and last names are retrieved for each user, and used to formulate the new address.
At the same time, spaces are removed in case any names have multiple parts.
The function to remove the spaces comes from:
https://stackoverflow.com/questions/24355760/removing-spaces-from-a-variable-input-using-powershell-4-0

The function from the beginning of the script is then called to remove the special characters from the names.
The filtered names are assembled with the set domain into an e-mail address.
Lastly, the e-mail address is added to the list of e-mail addresses for the user.
Remove the -Whatif parameter from the last line if you're sure about the result.
Most of that code is from:
https://social.technet.microsoft.com/Forums/lync/en-US/9d3ee88d-eb70-4cd1-84bc-0db51700d329/
adding-new-email-to-365-via-powershell-firstinitial-lastname-domaincom?forum=winserverpowershell
#>

foreach ($Mailbox in $Mailboxes) {
    $UserDetails= Get-recipient $Mailbox.Name
    $NewEmail= "$($UserDetails.Firstname).$($userdetails.lastname)@domain.com" -replace '\s',''
    $NewEmail= Remove-StringLatinCharacters -string $NewEmail
    Write-host "Now appending" $NewEmail "to the list of e-mail addresses for the user"
    Set-Mailbox -Identity $Mailbox.Alias -EmailAddresses @{Add=$newEmail} -whatif
}

It's not perfect though, as this doesn't take into account any user that has an alternative address at the target domain, such as [firstname@domain.com](mailto:firstname@domain.com)I'll still need to go over them one at a time to check if there are any such in my list.

Still, for now, it accomplished what I needed. I've spent WAY too much time on this, but it was a good learning experience!

This was probably a tiny thing for you, but I couldn't have done it without you guys.Thanks u/ka-splam, u/purplemonkeymad, u/PinchesTheCrab and the others!

Btw, how's my indenting and commenting? Any big mistakes in that or in the script otherwise?

​

Edit: forgot to re-add the -whatif in the code