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Does Sendy have to ability to be extended with plugins or are there plans for this?
Not at the moment but I'll consider this.
I think this would be great too. Especially if they worked similar to Wordpress with actions and filters:
Custom actions could be created that could say update a database entry when a user clicks a link in an email.
Custom filters could be created for things like modifying an email just before it's sent.
I'll add in my support for plugin development. One way to recoup the cost of adding it in would be to have a Sendy Plugin Marketplace - you could take a cut of any sales and purchases could automatically appear in your Sendy installation, making it easier for everybody all around.
I also support this feature.
I think plugins should be the easyest way for people to develop their own reports and other features, like an integrated SMS sender or extending the API for their own use.