Long story short: we had to migrate a shared customer support mailbox accessed by several people using Thunderbird/IMAP to a hosted Exchange Server 2019. Everything works fine... except the part where people need to label emails with different color categories, which depends on the IMAP server supporting it, and which Exchange's IMAP doesn't support (see https://stackoverflow.com/questions/3632102/imap-custom-keywords about how IMAP stores custom labels).
Now I'm investigating either installing the Owl plugin in Thunderbird or migrating everybody to Outlook 2016, so that people can use the native Outlook/Exchange categories. I'm performing tests between Outlook and a couple of Thunderbird/Owl installations, labelling emails on one computer and seeing how they appear in the others. (My Outlook doesn't have any custom categories, they are all as they are by default: "blue", "yellow", etc.). What I am observing is:
If I tag on Thunderbird an email with one of its default labels ("Important", "Work"...), the label shows up in Outlook, but without colors, and it appears in the "All categories" menu but labeled as "Not in the Master categories list".
If I tag in Outlook an email with one of its default labels ("Blue, "Yellow"...), the label shows up in Thunderbird... and it displaces one of the existing categories in Thunderbird: that is, "Blue" takes nº 8 in Thunderbird's labels list, and the labels that are behind it get displaced.
Based on this, I think that I can set up my users' clients so that they all have the same labels. However, for the sake of my curiosity (and to have a more complete picture of how everything works, in order to avoid problems in the future), I would like to know: how does Exchange Server store internally each email's categories? Is it a key-value pair, with a number and a text string, as I suspect? (Obviously colors are not stored). If so, which are the default numbers associated with the master categories in Outlook?
