$Addr= "IMCEAEX-_o=mydomain_ou=Exchange+20Administrative+20Group+20+25FUDITYHF23SPDLT+29_cn=Recipients_cn=Bobrikova+2C+20Olga@mydomain.ru"
$Repl= @(@("_","/"), @("\+20"," "), @("\+28","("), @("\+29",")"), @("\+2C",","), @("\+3F","?"), @("\+5F", "_" ), @("\+40", "@" ), @("\+2E", "." )) 
$Repl | ForEach { $Addr= $Addr -replace $_[0], $_[1] } 
$Addr= "X500:$Addr" -replace "IMCEAEX-","" -replace "@.*$", "" 
Write-Host $Addr

Добавить полученный адрес, как дополнительный кастом адрес:

1. X500 остаётся в левой части
2. Двоеточие ":" удаляется
3. правая часть начинается с "/имя домена"

Добавление/удаление на сайте Microsoft

Set-Mailbox "Dan Jump" -EmailAddresses @{add="dan.jump@northamerica.contoso.com"}
Set-Mailbox "Dan Jump" -EmailAddresses @{add="dan.jump@northamerica.contoso.com","danj@tailspintoys.com"}
Set-Mailbox "Dan Jump" -EmailAddresses "SMTP:dan.jump@contoso.com","dan.jump@northamerica.contoso.com","danj@tailspintoys.com"

Set-Mailbox "Janet Schorr" -EmailAddresses @{remove="janets@corp.contoso.com"}
Set-Mailbox "Janet Schorr" -EmailAddresses @{remove="janet.schorr@corp.contoso.com","janets@tailspintoys.com"}
Set-Mailbox "Janet Schorr" -EmailAddresses "SMTP:janets@contoso.com","janets@tailspintoys.com"

get-Mailbox username |select -ExpandProperty EmailAddresses | select proxyaddressstring

Set-Mailbox "Dan Jump" -EmailAddresses @{add="dan.jump@northamerica.contoso.com","danj@tailspintoys.com"}

Set-Mailbox "Dan Jump" -EmailAddresses "SMTP:dan.jump@contoso.com","dan.jump@northamerica.contoso.com","danj@tailspintoys.com"

Set-Mailbox "Janet Schorr" -EmailAddresses @{remove="janet.schorr@corp.contoso.com","janets@tailspintoys.com"}

Если адресов много, то можно копипастой вставить smtp адреса в скрипт и добавить пользователю с алиасом MyNewUser

$MyData = (@'
Myuser1@mydomain.ru
Myuser2@mydomain.ru
Myuser3@mydomain.ru
'@).split("",[System.StringSplitOptions]::RemoveEmptyEntries) 
 
foreach ($MyItem in $MyData)
{
$MyItem   
 
Set-Mailbox MyNewUser -EmailAddresses @{add=$MyItem}
 
}

1. Открываем $exbin\exchange.format.ps1xml
2. В раздел Get-Mailbox View\TableControl добавляем необходимые поля.

$a=(@'
test@mydomain.ru
'@).split("`n") |
 foreach {$_.trim()}
 
 foreach ($item in $a)
 {
     if (!(Get-Mailbox $item -ErrorAction silentlycontinue )){
        Write-Host -ForegroundColor Red $item #Not Exist
     } else {
        Write-Host -ForegroundColor green $item  #Exist   
     }
 }