1275571 - thunderbird deletes unread mail when set mail.server.default.dup_action to 1 (even if not really a duplicate message)

Status: UNCONFIRMED

(Thunderbird :: Untriaged, defect)

Description

[DELOUYA]

User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:32.0) Gecko/20100101 Firefox/32.0 Waterfox/32.0
Build ID: 20140903161535

Steps to reproduce:

set mail.server.default.dup_action to 1


Actual results:

When new mails arrive, a notification is issued as usual on the terminal.
If they are not opened at the moment, next time thunderbird checks the mail server for new mails, it deletes them!
Unread mails cannot stay longer than the mails server poll frequency!
If you are not in front of the terminal, you won't even know that someone emailed you!!


Expected results:

In my understanding, mail.server.default.dup_action is intended to suppress locally duplicated emails (which occurs frequently!), but having an unread mail both on the local client email side and on the server side does not appear as a duplicate mail to me!! 
Otherwise, at least, this must be clarified!
Rolling back mail.server.default.dup_action to 0 solves the issue..
Additional information: 
Thunderbird X64 version Daily 49.0a1 of 2016-05-08.
Terminal: Windows server 2008 X64
Configuration: 
- imap to exchange gateaway (corporate constraint that cannot change)
- inbox and sent mails are left on the mail server

Comment 1

[DELOUYA]

More additional info: 
Step to reproduce:
- send you an email from another client - gmail or whatever..
- look at the thnuderbird status bar
- at poll time - say 5mn frequency - 
  -- thunderbird checks the email server side
  -- loads the  mail
  -- send a system notification
- let the mail unread, don't move..
- 5mn later, thunderbird checks the email server side again
  -- deletes the unread mail - it tells it in the status bar!
- Not only, the mail disappeared from INBOX, but it is not anymore 
  accessible from the webmail
MOREOVER, how said duplicate deletion might delete both instances!!