The way the messages are sent is defined depending on the receiver
After selecting a receiver who has already connected to you through p≡p, you will see a green privacy status bar at the bottom of the window
No bar will be shown
Encryption can be disabled because of several reasons
There is an option to disable p≡p for a specific account. If p≡p is disabled for the current account, the privacy bar will show no color
When p≡p is disabled for an account and "keep decrypting" is set for the same account, p≡p will encrypt anyway when replying to an encrypted message
This button is in the composer toolbar. When the button is toggled, green and yellow privacy status will be shown with a lighter shade and the outgoing message will not be encrypted
You can access pEp options by clicking on the Thunderbird menu which looks like three bars on the top right of the window. Then click on Addons -> pEp for Thunderbird
Store messages securely
When this option is not checked, your mail will be saved unencrypted on your mail server
Warn if the conversation privacy is lowering
When you reply or forward a private message to untrusted receivers, the conversation will continue without encryption. In this case you will see a warning when this option is set
This option is designed to work with the disclaimer functionality provided by your mail server. You can choose "Only encrypted mail" in order for p≡p to add the disclaimer to encrypted messages that cannot be modified by your mail server.
Alternatively you can disable the disclaimer on your server and select "All mail" here to use only p≡p's disclaimer.