Add an email declaration & evidence type

But I'm less sure that there will be agreement on how to represent it.

Agreed. It's thorny.

I'm quite tempted to abandon it. The things that pricks my conscience is that a lot of cancellations will be email, but cancellation is the kind of boring thing that people don't want to optimise. It'd be nice if it just worked well enough OOTB.

Should we record the subject? What about the date? You mention the possibility of multiple emails, so that suggests multiple dates/subjects? Even the from/to email addresses might differ per email. So perhaps separate evidence for each email will work better? Is the body a plain text field or formatted? Might we prefer to save the email as an eml file (which seems much more convincing evidence IMO)? Or even export it as a PDF (which is already possible)? If the charity only has one email address, should that field really be mandatory?

I keep prevaricating. I will get back to you.

For evidence type: let's call it 'Email message file' and have a multivalue file field accepting .eml and .msg
People can make a seperate evidence type for less forensically robust evidence if they want.

For declaration type:

This is complicated by the fact that I'm suggesting evidence of email confirmation in #3537036: Written confirmation.

We already have two cases where we programmatically create evidence. They use an evidence type and fields that are locked, so they can't be deleted, and this type is not available to staff or admins. We should do the same. It keeps the customisable settings nice and separate from the config that the module depends on.

Ah good! I was wondering about exactly that. This helps a lot.

OK. The point is to make something that is good enough to support the unglamous but necessary admin work of cancellation without having to be designed carefully by sitebuilders in advance. So let's aim for breadth of relevance combined with relative simplicity:
(1) field_charity_email 'Email of the charity' 'The email address where the charity received the message.'.
(2) field_donor_email 'Email of the donor' The email address that sent the message to the charity'.
(3) field_text. 'Email text' The text of the email(s) exchanged with the donor.
(4) field_file 'File' A file containing a record of the email(s). Multivalue file field, allows eml, msg, txt, pdf, jpg, jpeg, png, webp. [Yes this duplicates other evidence types, but sometimes you might want to record the to/from email addresses alongside a screenshot that doesn't capture that info.]
No fields are required, all are visible by default on the form.

This makes a solid base that should be good enough for almost any site to use OOTB, doesn't force awkward choices on staff, while easy for sites to streamline if they want to be particular about how emails are recorded.

Maybe there's a better way of handling this, but I can't figure it out and this seems good enough.