@helebor . Certificate errors normally occur between a client and the mail server. When that happens, no emails are sent or received.
Considering your emails that aren't sent, is there a specific domain that fails or multiple domains?
Are the failures consistent? A particular recipient will either always work or always fail?
From reading the product help, it seems like the client can encrypt the data part of emails using a key specific for each contact address, or, optionally, not encrypt. And, the recipient must have a matching key to decrypt.
Might be easier to ask The Bat! support about the error message. It shouldn't matter that your version might be unsupported. The error message has a more detailed meaning in relation to "address book".
