You can mail-enable multiple accounts with a single Powershell command. Look below for some examples:

Mail-enable AD accounts whose first name is John:

Get-ADUser -Filter * | Where {$_.GivenName -like "John"} | ForEach-Object { Enable-Mailbox -Identity $_.DistinguishedName }

Mail-enable all accounts in an OU called Engineering:

Get-ADUser -Filter * -SearchBase "OU=Engineering,DC=contoso,DC=local" | ForEach-Object { Enable-Mailbox -Identity $_.DistinguishedName }