Wrong e-mail address in `From` field with non-ascii characters

This issue has been migrated from [#10173](https://github.com/matrix-org/synapse/issues/10173).

---

### Description

I set the
`
notif_from: "%(app)s üzenet "
`
and I end up with:
`From: "Matrix üzenet "@maildomain.example.com`

The weird thing is that the:
`Return-Path: `
is correct.

### Steps to reproduce

- set the `notif_from:` to something like in the documentation
- restart synapse
- post message and wait for the e-mail notification

I expect to see `Matrix üzenet ` in the **From** field of the incoming mail.

### Version information

- **Homeserver**: Synapse 1.35.1

If not matrix.org:

- **Version**:

- **Install method**:

- **Platform**: