FTP and SMTP, unlike Http, are sequential. This means that requests are handled one after another.
It is true that if you have code such as:
For i = 1 To 100
SMTP.To.Add(...)
SMTP.Sent
Next
Then it is possible that one message will be sent before the other.
As they are sequential, there is no benefit for sending them like this. You can instead send the next message in the MessageSent event.
There is no object similar to HttpJob here that a tag can be added to it. It is possible to add a Tag parameter to all the methods but I think that it is a bit too much.
It shouldn't be difficult to create a solution similar to HttpJob and HttpUtils2Service for FTP or SMTP. I'm not sure that it will be too useful.