Feature: Allow to set the tag attribute on Postal messages

[a47ae]

Postal allows to tag messages send via their API. This would be a useful feature to add to this package.

[willpower232]

This is an interesting idea, does Laravels MailMessage have any concept of tags or should we provide an interface to Postals SendMessage?

(an interface to Postals SendMessage may also be handy for custom headers for #8)

[a47ae]

So I researched a bit how other popular mail driver packages handle this and I saw two solutions. The sendgrid package uses the embedData method on the Illuminate\Mail\Message class to provide custom data. This is, in my opinion, not the cleanest solution, but the other solution would require not using the Notification class, but rely on a custom class like the mailgun package does.

[Josh-G]

On hold for now due to outstanding issues in #8