I have a fully patched Exchange 2013 server and the latest Thunderbird IMAP-client.

When the user tries to open some subfolders under inbox he gets an error "The current operation on 'subfolder' did not succeed. The mail server for account user@Exchange responed: Server Unavailbale. 15."

Some subfolders work though and subfolders under the subfolder that cant be opened works.

I found this post but no answer to what the user changed to make it work:

https://social.technet.microsoft.com/Forums/office/en-US/2508f50f-6b28-4961-8e6c-5425914d4caa/no-server-unavailable-15-on-exchange-2013?forum=exchangesvrclients

The folder is visible and readable from other clients like Outlook and webmail.

• Edited by Harmankardon 21 hours 6 minutes ago

A session log says the following:

* OK The Microsoft Exchange IMAP4 service is ready.

A01 LOGIN username password

A01 OK LOGIN completed.

A02 SELECT INBOX

* 1202 EXISTS

* 0 RECENT

* FLAGS (\Seen \Answered \Flagged \Deleted \Draft $MDNSent)

* OK [PERMANENTFLAGS (\Seen \Answered \Flagged \Deleted \Draft

$MDNSent)] Permanent flags

* OK [UIDVALIDITY 14] UIDVALIDITY value

* OK [UIDNEXT 237078] The next unique identifier value

A02 OK [READ-WRITE] SELECT completed.

A03 SELECT Subfoldername

A03 NO Subfoldername doesn't exist.

A04 SELECT INBOX/Subdfoldername

A04 NO Server Unavailable. 15

A session log says the following:

* OK The Microsoft Exchange IMAP4 service is ready.

A01 LOGIN username password

A01 OK LOGIN completed.

A02 SELECT INBOX

* 1202 EXISTS

* 0 RECENT

* FLAGS (\Seen \Answered \Flagged \Deleted \Draft $MDNSent)

* OK [PERMANENTFLAGS (\Seen \Answered \Flagged \Deleted \Draft

$MDNSent)] Permanent flags

* OK [UIDVALIDITY 14] UIDVALIDITY value

* OK [UIDNEXT 237078] The next unique identifier value

A02 OK [READ-WRITE] SELECT completed.

A03 SELECT Subfoldername

A03 NO Subfoldername doesn't exist.

A04 SELECT INBOX/Subdfoldername

A04 NO Server Unavailable. 15

Found a workaround for the problem.

If the user copied the mail (via another client or webmail) to another folder and then removed the old folder. Recreated it and then moved the mail back he could see the mail in the folder again.

• Marked as answer by Harmankardon 21 hours 3 minutes ago