There are no Pro or Plus versions as there are with BlueMail and Mailspring, and it's not extensible like Thunderbird. However, it is a competent if no-frills personal information manager for handling email and calendaring tasks, managing a contacts list locally or online, and subscribing to feeds. If you maintain a local address book, you can even expose its contents as a data source for use in LibreOffice, which could be useful when running a mail merge.