Ensure task mail respects `[scheduler][mail]to/from`

[MetRonnie]

Closes #5976

Plus tidy some cruft I spotted along the way (sorry, couldn't resist)

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] No dependency changes
• [x] Tests are included
• [ ] CHANGES.md entry included if this is a change that can affect users
• [x] No docs entry needed
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.