## What?
Updates the SMTP envelope sender for outgoing mail.
Matcha now uses `send_as_email` for `MAIL FROM` when it is configured.
If it is not configured, it keeps the existing fallback behavior.
This applies to regular email sending and calendar replies.
## Why?
Closes #1274
Previously, `MAIL FROM` was based on `fetch_email`, with `account.email`
as the fallback.
Now the order is:
```text
send_as_email -> fetch_email -> account.email
```
`fetch_email` is used to match incoming messages to an account. For
outgoing mail, the configured Send-As address should be used first.