I've been using the BTmail app for many months with no issues on the Huawei with Android. The last month whenever I send an email that contains a jpeg attachment then that email will not show in the "sent" folder. In fact it will not show in the "sent" folder of my email account on any other device either, but it will always have been received by the recipient. At the same when preparing an email many of the contacts that I want to mail to no longer display their correct email address, for example, if its FREDBLOOGS123x123@GMAIL.COM and I click on that in the contacts it will come up in the "To" section of the email as "FREDBLOGGS-TIMESTAMPXXXXXXX@" where "x" is a series of numbers.
I've reinstalled the app several times and my phone is up to date. I can send the same email using my Google Pixel 3 and do not have that problem.
The Huawei comes with its very own email app and that works fine. Unfortunately with the Huawei app I can't move my emails to other folders.