Yup, until they receive a court order asking them to mitm an inbox, if they haven't already...
This entire system of "receive email in clear text but store it encrypted at rest" is smokes and shadows, really.
The former has distinctly less legal requirements than the latter, and authorities might be OK with keeping it that way, as metadata is already good enough in most cases.
It wouldn't technically be a MITM attack, they would just capture the incoming email. Tuta was famously forced to do that once by the German authorities.
https://dwheeler.com/trusting-trust/
There are also a number of people making minimal OSes, interpreters, and compilers that you can, for example, assemble by hand and type in "from scratch".
There was a nice list of those that I can't find right now, but you could look at
https://bootstrappable.org/projects/mes.html
as one example in this direction.
To make a perfectly secure system, the first step is to obtain high purity sand.
On the bright side, it's hard to imagine that many of these attacks will be self-propagating, which is the particularly insidious thing about the Trusting Trust attack. Yes, hardware is used to design hardware, but typically in a more indirect and heterogeneous way than the "compiler compiling itself" scenario. To be concrete, I'd say Microsoft or Canonical has much more to fear from a Trusting Trust sort of attack than Intel does, but the software developers also have better options to contain or detect such an attack.