Richard,
It seems to me that groups.io has a rather inconsistent story
regarding privacy of email addresses.
That's true, the inconsistencies and the feature itself are largely borrowed from Yahoo Groups tradition.
It appears to be the case that if you receive posts via email, you can
often?/always? see the poster's email address, regardless of whether
you have access to the member list.
Always.
A fundamental decision in the design of the email support is how much modification to do inside message bodies. For email message delivery the answer has always been "as little as possible". I think at present it is limited to appending the group footer and (at the receiving member's option) replacing attachments with links to stored copies of the attachments.
I think this is a good answer, on both efficiency and integrity grounds.
Display of messages on site, and the inclusion of messages in Digests, have further processing. Material quoted from prior messages, if detected, is hidden on site under an ellipsis button and is not included in Digests. Email addresses in the message body, excluding those in the groups.io domain (and maybe some few others) are truncated ("fig-leafed") when shown to members, but not when shown to moderators/owners.
If you then reply to a post and quote the original message, that
quotation will contain whatever your email client put in the 'on such
and such a date so and so wrote' bit,
As it turns out, email services vary considerably in how they preface such quotes in replies. Recognizing them is an on-going update item for Groups.io.
and this seems to depend on whether the sender has defined a display
name in their profile.
Email services are idiosyncratic about this. But generally yes, many will include only the "Display Name" portion of the From address, if there is one; some will include both Display Name and email address, a few will include only the email address.
If there is no display name, groups.io shows the whole email address,
not truncated in the same way as it does for replies via the website.
In email, and when viewed by moderators/owners on site, whatever the sender's service put into the message is shown. For members viewing on site Groups.io truncates the address (again with the exception of groups.io addresses).
It seems to me that a possible improvement would be for groups.io to
assume a display name of the first part of the email address for
posters who haven't explicitly defined a display name.
Again, if you're talking about the preface to a quote, that comes from the sending member's email service.
Otherwise that's pretty much what Groups.io already does: the truncated address ("user@...") is shown in place of the Display Name.
Not sure what can be done about the full email address not being
visible in the email client, whilst staying compliant with the
standards.
Yes, it would be possible. As you noted, Yahoo Groups' "hide" feature is one example implementation; a feature which is available to members of GMF's sister Yahoo! group.
Several approaches to this have been discussed in beta@, but nothing has come of it so far. If you're curious, I think "anonymous" would be a good search word, as in groups with anonymous members.
Shal
Shal Farley