Is there a developer API for AddIns?

we developed and maintain an AddIn for MS Outlook and Thunderbird. It is for encryption purposes.

I’m interested in also supporting eM Client. Is there a way to develop third party AddIns to eM Client? And if yes, is this API powerful enough to manipulate an email before it is sent (read/save/remove/replace attachments and body)?

eM Client already has encryption capability for sending and receiving messages. Please see out Blog entry for how to do that.

Unfortunately addons are not supported.