Client connections that want to use `STARTTLS` on a port 143 plaintext connection tend to require a `STARTTLS` capability to be presented on an otherwise plaintext connection, such that the connection can be explicitly upgraded.
Most IMAP client software actually require the capability to be present, or do not allow the connection's encryption to be negotiated.
#guam does not currently include the `STARTTLS` capability in its initial `CAPABILITY` response.