send a separate mail to each recipient, rather than all at once

[karenetheridge]

when sending mail to multiple recipients (e.g. when adding a build to an organization and we notify every organization member) instead of sending a single email with all recipients in the To: header (which can leak membership information) loop through each recipient individually and send a separate message.

this isn't a huge deal since in most (all?) circumstances members can see organization membership anyway, it is still cleaner to keep the recipients separate.