==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 0.9.9
- Deployed with iRedMail Easy or the downloadable installer? downloadble
- Linux/BSD distribution name and version: Ubuntu 16.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): mysql
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? Yes
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

I have a strange problem with one of client mailbox. 6days ago outlook doesn't want to sync his email.
Via Iphone working good. Via imap working good. Via SOGo web working good.
I'v try to delete and recreate account in outlook (2016,2013). Inital connection seems to be ok. (green pipe checked)
But after that when it want to download messages and folders waiting for 1 min and !no connection  on the status bar.
I've check sogo log files on debug mode, but there is no any error.
I'e found one strange line in apache log file:

POST /Microsoft-Server-ActiveSync?Cmd=FolderSync&User=xxxx.yyyy@domain.com&DeviceId=5600F579B7FD4831AAD1B30EA0DF2561&DeviceType=WindowsOutlook15 HTTP/1.1" 501 370 "-" "Outlook/15.0 (15.0.5125.1000; MSI; x64)

Another strange thing. Same machine with another account working properly.
POST /Microsoft-Server-ActiveSync?Cmd=FolderSync&User=zzzzz.oooo@domain.com&DeviceId=9FE35E0F8C3247CAB98871A2150CEE44&DeviceType=WindowsOutlook15 HTTP/1.1" 200 4032 "-" "Outlook/15.0 (15.0.5125.1000; MSI; x64)

What can I do now?
Thank you!
Szabolcs