Add support for 3rd party APIs such as mailgun

[adamlesniak]

Subject is self explanatory I believe it would be nice feature and we could standardise this module.

[fritx]

Agreed, what could we do next?

[justin713]

Looks easy enough, although I do believe Nodemailer would need to (and should) be upgraded up to its most recent release. The createTransport() method needs to be adjusted to accept any arbitrary transport configuration.

Currently, the transport is hard-coded to SMTP:

// index.js
var transporter = nodemailer.createTransport('SMTP', options.smtp);

Nodemailer can be made to use any arbitrary transport that implements at least one send() method and two properties, name and version. (See Nodemailer docs regarding transports.).

So, one could have the following:

// index.js
 var transporter = nodemailer.createTransport(transport);

...where transport is a valid transport object. In the case of Mailgun, there's already a module for that: nodemailer-mailgun-transport.

So, if gulp-mail accepts a transport object as an argument, users will have control over which transport they use.