Add support for forwarding via SMTP (with SRS due SPF)

[mback2k]

[brpaz]

Hey.

First of all, congrats on this amazing project. As a Go developer myself, really like to see how it was implemented.

But I think I will need this feature.

I am using your tool to forward emails from an old provider who doesn't support forwarding, to my new domain hosted on Zoho Mail.

The problem is the email filters are not applied to the emails forwarded using your tool. I guess that´s a Zoho limitation. didn´t test with other providers like Gmail. I have created a support ticket on their end also.

Still, I believe sending the emails via SMTP might be a workaround and solve the problem.

If I cant solve the problem and as I couldnt find decent alternatives, I will try adapt your code to support SMTP. I will do a "dirty" proof of concept before and if everything works with my use case, I try to submit a PR.

This library is interesting and is made by the same guy of go-imap..