Within Stello there is the Editor Guide with all the essential information you'll need. In addition to that, some topics that aren't covered are:
Advanced text formatting
In addition to the formatting you can achieve with the buttons available, you can also create:
Create a section divider by typing
--- like this:
To start a new line without starting a new paragraph, type
See How do I ...?
Stello messages are opened by clicking a link. Invitations are the templates used to invite recipients to open your message. The invitation used for email recipients is always displayed above the message in the editor and can be customised per message. You can also customise the default in the sending profile settings.
The image in email invitations is encrypted and will expire when your message does, where as the text will not and will stay in your recipients inbox until they delete the email themself (probably never).
Non-email contacts must be sent the link by copying and pasting it into another messaging platform such as social media. A brief message is also included above the link and this can be customised as well.