[sirinsidiator]

I added the following code to one of my addons:

saveData.mailLog = saveData.mailLog or {}
    local function log(message)
        saveData.mailLog[#saveData.mailLog + 1] = ("%d.%d: %s (%d)"):format(GetFrameTimeSeconds(), GetFrameTimeMilliseconds(), message, GetInteractionType())
    end
 
    ZO_PreHook("RequestOpenMailbox", function()
        log("RequestOpenMailbox")
    end)
    ZO_PreHook("CloseMailbox", function()
        log("CloseMailbox")
    end)
    ZO_PreHook(MAIL_INTERACTION_FRAGMENT, "Show", function()
        log("MAIL_INTERACTION_FRAGMENT:Show")
    end)
    ZO_PreHook(MAIL_INTERACTION_FRAGMENT, "Hide", function()
        log("MAIL_INTERACTION_FRAGMENT:Hide")
    end)
    RegisterForEvent(EVENT_MAIL_OPEN_MAILBOX, function()
        log("EVENT_MAIL_OPEN_MAILBOX")
    end)
    RegisterForEvent(EVENT_MAIL_CLOSE_MAILBOX, function()
        log("EVENT_MAIL_CLOSE_MAILBOX")
    end)
    RegisterForEvent(EVENT_MAIL_INBOX_UPDATE, function()
        log("EVENT_MAIL_INBOX_UPDATE")
    end)

Then I did the same thing that Baertram just posted and opened the mailbox via keybind right after I logged in, but the mails show up without an issue for me.

Here is the log:

[1] = "89.89414: MAIL_INTERACTION_FRAGMENT:Show (0)"
[2] = "89.89414: RequestOpenMailbox (0)"
[3] = "93.93555: EVENT_MAIL_OPEN_MAILBOX (0)"
[4] = "93.93555: EVENT_MAIL_INBOX_UPDATE (0)"
[5] = "93.93555: EVENT_MAIL_INBOX_UPDATE (0)"
[6] = "93.93555: EVENT_MAIL_CLOSE_MAILBOX (0)"
[7] = "94.94383: EVENT_MAIL_INBOX_UPDATE (0)"
[8] = "98.98023: MAIL_INTERACTION_FRAGMENT:Hide (0)"
[9] = "98.98023: CloseMailbox (0)"
[10] = "98.98049: EVENT_MAIL_CLOSE_MAILBOX (0)"
[11] = "510.510312: MAIL_INTERACTION_FRAGMENT:Show (0)"
[12] = "510.510312: RequestOpenMailbox (0)"
[13] = "510.510414: EVENT_MAIL_OPEN_MAILBOX (7)"
[14] = "510.510414: EVENT_MAIL_INBOX_UPDATE (7)"
[15] = "580.580472: MAIL_INTERACTION_FRAGMENT:Hide (7)"
[16] = "580.580472: CloseMailbox (7)"
[17] = "580.580486: EVENT_MAIL_CLOSE_MAILBOX (0)"
[18] = "580.580486: EVENT_MAIL_CLOSE_MAILBOX (0)"
[19] = "851.851624: MAIL_INTERACTION_FRAGMENT:Show (0)"
[20] = "851.851624: RequestOpenMailbox (0)"
[21] = "851.851777: EVENT_MAIL_OPEN_MAILBOX (7)"
[22] = "852.852526: MAIL_INTERACTION_FRAGMENT:Hide (7)"
[23] = "852.852526: CloseMailbox (7)"
[24] = "852.852553: EVENT_MAIL_CLOSE_MAILBOX (0)"
[25] = "852.852553: EVENT_MAIL_CLOSE_MAILBOX (0)"