I don’t have Obsidian around, but this has been happening elsewhere lately too, almost certainly because of this underlying Electron issue: https://github.com/electron/electron/issues/43819
Unfortunately there’s not much you can do about it. Electron decided to depend on functionality not yet in a released version, and that very interesting choice flows down to everything that updates their Electron on the regular.
The email ecosystem is changing in recent years but yeah, it’s best to expect that there is at least one opportunity for any given email to be sent over the internet unencrypted. MTA-STS has been slowly changing the landscape but adoption isn’t going all that great.