A Soup Mail user might wish to see people that are Blind Carbon Copied (BCC'd) in an email within Soup Mail. Soup Mail doesn't process the BCC field from emails, and the presence of the BCC field in an email downloaded from Soup Mail is not guaranteed.
Blind Carbon Copy (BCC) is an attribute of an email that is relative to the person looking at the email. That is, the BCC field is present in the email sent by the sender, but is not necessarily present for a recipient of that email. Hence, there is an inconsistency as to whether BCC is even available, for the same email. This is not something Soup Mail is doing, rather it’s part of the implementation by mail services of the RFC822/RFC2822 standards (see e.g. RFC822 section 4.5.3/RFC2822 section 3.6.3). In the vast majority of cases, mail services do not include the BCC field for recipients of an email (so that recipients are not aware of who may have been BCC'd in the email they're looking at).
In terms of how this affects Soup Mail, if the email is saved to Soup Mail by its sender, the BCC field is present in the actual email saved (and therefore can be seen if the email is downloaded from Soup Mail), but if it’s saved by a recipient, it’s simply not present.
Soup Mail cannot therefore purport to have that information available, because we cannot guarantee that if there were BCC recipients, that we even know that is the case. Users relying on that information (e.g. for ‘emails I’m involved in’ filtering) would likely only get a more or less random experience in terms of whether it’s present or not, so it can’t be relied upon. Given this is the case, we’ve not implemented BCC in Soup Mail, and don’t plan to.