@Tungro I'm really sorry this stems from my doing. Quite honestly, I'd done it to /avoid/ cert change issues, and you're the first to report any problems. Do let me know if you need me to put it back.
Alternatively, if you're considering a new client that I'd consider better (I know this is personal opinion), do take a look at K-@: https://play.google.com/store/apps/details?id=com.onegravity.k10.free&hl=en_GB
Don't worry! It's not your fault at all that the app is not coded to handle something so simple. You totally did the right thing by changing the certificate to one with a chain with a trusted CA. If it was a problem with what you did, I'd have come to you. But the problem lies with the email client.
Thanks for the recommendation, you're the second to have mentioned that one. I'm trying out Outlook first as apparently their Android client is actually really good. Will report back!