I'm a little confused
The description of the error in the release notes is ambiguous.
" Even when marked to be encrypted, an email with only Bcc recipients is sent in plain text!"
Does this mean that the bug only shows up & sends mail unencrypted to the BCC recipients when the BCC field is used but the TO & CC fields are blank, or does this mean that the bug always sends mail unencrypted to the BCC recipients independent of the status of the TO & CC fields?
These are two very different behaviors with two very different probabilities of being triggered due to the way people often use BCC. My expedience with BCC both sending and receiving email is that the TO field is always filled with some address, sometimes a dummy address (such as the sender's). If the "only Bcc recipients" requirement is in fact strictly the case, then most instances of using BCC that I've seen would not trigger the bug.
This just underscores how describing bugs needs to be done with precision.