Identifier for accounts in message list

[mat-m]

Is your feature request related to a problem? Please describe.

Following #7513 , I'm opening the Feature Request.
FTR, we had a color marker until Mail 2.0.3, but it was removed with #6981. In the 7513 thread, there was no rationale beside the fact it did not fit with the new round theme. Jan provided some arguments about not using colors.

Actually color is one option, but a text identifier could also be used. K-9 lately uses a combination (2 first chars of domain with the color to split between account on the same provider).

My use cases where the account color marker is useful:

• Some phishing are very convincing, so identifying easily that this kind of mail should not be in this mail account help a lot.
• When you're sorting out recent emails, having the color helps to triage
• When you're searching for an email in a known period, without being sure which account it's in (searching in 2 accounts out of 7)
• Mail deleted too quickly. It helps to know which trash we should search in

About scam/spam/phishing :

Some examples:

• When a Paypal mail is arriving on Account B, I can reject it as phishing since it is not used for Paypal, without even opening it; but I need to know it went to Account B
• On a well-crafted scam email, I need to search which account it targeted. If recipients were in BCC, I can only resort to open the source and check the headers, or loop on all accounts to see where it is stored.
• It's also like a first-level filter for junk. That's how my brain works. I do not pretend everyone with multiple accounts will need it.

About identifier for account:

I actually do use K9-Mail and its color identifiers 😉 . I  even chose the colors myself in K-9 to know the match between account and color.
For imported accounts in gmail, I actually defined some labels to identify them. I'm that type of user. 

More on my use cases of identifiers:

That's how my brain process information, and I was relieved to find a match in NC Mail at first 😄
I don't expect you to find alternatives in NC mail, but maybe it will provide other views on NC Mail usage, and hopefully support my request to keep colors.

• The identifier tells me also where to move the mail (should I need to scroll the account list or not) to put it in another folder
• The identifier tells me also where to search for other mails from the same sender in a preset folder (often faster than search, although it improved recently)
• Be in the unified inbox to see all new/unprocessed mails at once. Checking in the same time if I received the expected mail on Account C is easier with visual identifier than checking against the sender (which I may not know with precision, it could be just a domain origin, etc.) .
• Helper to differentiate origin, for example: Two orders went to Account A & B. Same carrier. Mails are now often not mentioning the shop, so knowing the DHL mail arrived into Account A is time-saving (even more on Christmas time 🎄 ).

Describe the solution you'd like

I understand that my usage model may not be the heart of the target for NC Mail, but it would be nice to have a way to keep account identifiers and productivity improvement they bring, at least through Theming Custom CSS. 
Having visual identifiers with a size of zero, or display:none, and being able to activate them for whoever wants it. My guess is that the users that finds value in colored accounts are mostly willing to get their hands dirty for this.

There are at least 3 places to add an account identifier in NC Mail message list:

My opinion is that the identifier is more useful in the messages list to differentiate them, but at least we can have it in the preview pane (in front of the thread subject, maybe)

Naïve question: About NC Mail usage models, I'm curious to know whether you have personas to describe redline scenarios & behaviors within the app ?

For accessibility purposes, color identifier, as an item, can have an alternate text or a legend or label with the full account name.

Describe alternatives you've considered

No response

Additional context

No response