I tried Deltachat to communicate with an Enigmail enabled Thunderbird client to test the encryption.
Unfortunately, whilst this was successful and messages were encrypted / decrypted at each end, when replying from Thunderbird, it used the decrypted text as the subject line of the reply, which obviously remained in plain text when returned to the Deltachat server, ie the content leaked! The body content though was encrypted properly, as it should be. Thus it appeared to the Deltachat user that the channel was secure, which it was, as the lock icon was shown.
Deltachat to deltachat doesn’t suffer this problem as it keeps the email subject line empty.
TLDR anyone sending encrypted messages from Thunderbird must manually remove subject lines from every reply email to ensure previous messages don’t become visible on the email server.