The configuration UI is rather limited at the moment I'm afraid. I'll reproduce some information here from a recent mail exchange that should give you a starting point at least:

The configuration for this is somewhat obscure so far I'm afraid;

• For SMTP anything starting with smtps:// and a port that is *not* 465 will end up using starttls
• For IMAP you will have to add a starttls=true entry to the resource config file by hand (as there is no UI for this) To find the right configuration file use "sinksh info" to find the config location and "sinksh list resource" to find the resource identfier (the confguration file will have the same name).

> You have to enter the flatpak environment before you can execute anything:
> [I] ⋊> flatpak run --command=/bin/bash com.kubeproject.kube
> mollekopf ~ $sinksh info
> ...
> Config location: /home/mollekopf/.var/app/com.kubeproject.kube/config/sink
> ...
> mollekopf ~ $sinksh list resource
> Resource | Identifier                           | account                              | server                        | type              
>          | 887f4e0b-9cc1-4f4e-8e1b-4e0294deeae3 | fbe4df3b-acad-40b6-af56-d4d0cfc9c1c0 | imaps://beta.kolabnow.com:143 | sink.imap         
> ...
> mollekopf ~ $ls /home/mollekopf/.var/app/com.kubeproject.kube/config/sink/\{887f4e0b-9cc1-4f4e-8e1b-4e0294deeae3\}.ini 
> You will have to edit the file from outside flatpak unless you like nano.

If that doesn't help or doesn't work, let me know.

Sooner or later the config UI will just have to be fixed.

Thanks for the update. I think I'll wait until Kube has the interface,
until then Thunderbird is working OK for me.