The overall API type signature is not consistent
In `Mailer` we have 
* `sendMail()():Try[Unit]`
* `sendMails()():Try[Seq[Try[Unit]]]`
In `AsyncMailer` we have
* `sendMail()():Future[Unit]`
* `sendMails()():Future[Seq[Try[Unit]]]` 

Shouldn't we have something along the lines of 
In `Mailer`
* `sendMail()():Try[Unit]`
* `sendMails()():Seq[Try[Unit]]`
In `AsyncMailer`
* `sendMail()():Future[Try[Unit]]`
* `sendMails()():Future[Seq[Try[Unit]]]`