Comment 36 for bug 603402

Do you enable IDLE command use?
If yes, same problem as bug 512745.
  bug 512745 :
    Gmail Label of A and B is added to a mail, and has flag \Seen(or no \Seen).
    The mail is shown in both Gmail IMAP folder named A and B,
    and Tb knows flag \Seen(or \no \Seen) of both mails.
    Mail in Gmail IMAP folder named A is changed to no \Seen(or has flag \Seen).
    Mail in Gmail IMAP folder named B is also changed to no \Seen(or has flag
    \Seen) at Gmail IMAP server, because both are copy of same mail each other.
    This flag status change at Gmail IMAP folder named B is not notified to
    IMAP client like Tb via IDLE.
  Your report :
    A mail in a Gmail IMAP folder has flag \Seen(or no \Seen).
    Tb1(or Smart Phone etc.) and Tb2 knows flag \Seen(or \no \Seen) of the mail.
    The mail is changed to no \Seen(or has flag \Seen) by Tb1(or Smart Phone).
    The flag status change at Gmail IMAP folder is not notified to Tb2 via IDLE.
These are same phenomenon.

> I have checked to see whether those emails are flagged as read or unread
> but using the gmail web client and they are indeed being marked as read
> but Thunderbird will not refresh their read/unread status.

If the mail is "a mail in a conversation of Gmail which contains multiple mails", phenomenon of bug 478996 occurs. Don't use "read/unread status of a conversation at Gmail Web interface" as evidence of "read status or unread status of a mail", unless it's sure that the conversation of Gmail has single mail only or that all mails in a conversation of Gmail has same status of read or unread.

See also bug 518581, if Gmail IMAP.

> Expected results:
> Each time Thunderbird attempts to get mail it should check the status of read
> and unread emails from the IMAP server and mark those emails as read if applicable.
> The only way to force TB to refresh the read/unread status is
> to completely close TB and re-open which is obviously very annoying.

To avoid excess network traffic due to "fetch of flags of all mails in IMAP folder upon each folder access",
(imagine 1,000,000 mails in a folder * 10,000 folders, new mail check of all mails folders each one minute)
Tb currently relies on "flag status change notification from server via IDLE", but many IMAP servers including Gmail IMAP unfortunately don't look to do so.
Following is a possible workaround at Tb in your case, if your any IMAP folder is never huge.
  - Disable IDLE command use
    Max chached connections = 1
    (I think Smart Phone's setting is similar to: no IDLE, max=0)
    (Tb doesn't permit max=0. Always uses default of 5 if max=0 is specified)
  - When you want to know flag status change by other IMAP cilent,
    or when you want to know flag status change propagation to
    same mail in other Gmail IMAP folder,
    click different folder, then click needed folder at folder pane of Tb.
Can you see flag status change by other IMAP client like Smart Phone, using Tb without restart?