How do you trust someone else to manage your mail service?
How do you trust every line of an open source package without auditing it yourself?
In your hierarchy of risk/trust, this one is pretty small.
Is this rhetorical?
How can you trust any app that has access to your data?
Send / receive encrypted messages. Print out encrypted data. Type into computer you built yourself from individual transistors to do the decryption.
The same way you trust your surgeon. How do you know he will make you better and not kill you in an elaborate way?
How do I know you are a real person and not a figment of my imagination?
Can you prove that you exist?
I don't use non-open-source, third party email clients.
For G Suite (personal) - I use gmail web client.
For Office 365 (work) - I use Outlook.
For my own mail server - I use Thunderbird or forward to gmail.