==== Required information ====
- iRedMail version (check /etc/iredmail-release): 0.9.7 MARIADB edition.
- Linux/BSD distribution name and version: Debian 9 Stretch
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? Not yet
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Hi ZhangHuangbin 

I've recently started using iRedMail on a test site, and I'm seeing lots of 499 and 502 errors from Nginx:

--------------------- httpd Begin ------------------------

Connection attempts using mod_proxy:
    51.15.146.5 -> www.msftncsi.com:443: 3 Time(s)

Requests with error response codes
    400 Bad Request
       www.msftncsi.com:443: 3 Time(s)
    499 (undefined)
       /Microsoft-Server-ActiveSync?Cmd=Sync&User ... =SAMSUNGSMG389F: 621 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Sync&User ... =SAMSUNGSMG390Y: 310 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =SAMSUNGSMG389F: 300 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... iceType=Android: 194 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Sync&User ... =SAMSUNGGTS7710: 147 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =SAMSUNGSMG390Y: 131 Time(s)
       /Microsoft-Server-ActiveSync?User=xyz@ ... iPhone&Cmd=Ping: 102 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... =SAMSUNGSMG389F: 100 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... =SAMSUNGSMG388F: 51 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... =SAMSUNGSMG390Y: 45 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =samsungSMG900I: 37 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... =SAMSUNGGTS7710: 29 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Sync&User ... =SAMSUNGSMG388F: 29 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... iceType=Android: 24 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =SAMSUNGSMG388F: 8 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=OPTIONS&U ... =SAMSUNGGTS7710: 1 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Search&Us ... iceType=Android: 1 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Sync&User ... iceType=Android: 1 Time(s)
       /Microsoft-Server-ActiveSync?User=xyz@ ... iPhone&Cmd=Sync: 1 Time(s)
    502 Bad Gateway
       /Microsoft-Server-ActiveSync?Cmd=Sync&User ... =SAMSUNGSMG389F: 158 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =SAMSUNGSMG389F: 84 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Sync&User ... =SAMSUNGGTS7710: 78 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Sync&User ... =SAMSUNGSMG390Y: 70 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... iceType=Android: 38 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =SAMSUNGSMG390Y: 30 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... =SAMSUNGSMG389F: 25 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =samsungSMG900I: 21 Time(s)
       /Microsoft-Server-ActiveSync?User=xyz@ ... iPhone&Cmd=Ping: 18 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... =SAMSUNGSMG388F: 12 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... =SAMSUNGSMG390Y: 10 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Sync&User ... =SAMSUNGSMG388F: 8 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =SAMSUNGSMG388F: 7 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... =SAMSUNGGTS7710: 4 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=FolderSyn ... iceType=Android: 3 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Search&Us ... iceType=Android: 1 Time(s)
    504 Gateway Timeout
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =SAMSUNGSMG389F: 1 Time(s)
       /Microsoft-Server-ActiveSync?Cmd=Ping&User ... =SAMSUNGSMG390Y: 1 Time(s)

---------------------- httpd End -------------------------

There is only one error-type in the Nginx logging, though - all the same as this:

2017/11/19 18:54:12 [error] 521#521: *101616 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: {ip-addr}, server: _, request: "POST /Microsoft-Server-ActiveSync?Cmd=Sync&User={email-addr}&DeviceId={stripped}&DeviceType=SAMSUNGSMG390Y HTTP/1.1", upstream: "http://127.0.0.1:20000/SOGo/Microsoft-S … UNGSMG390Y", host: "{domain-name}"

I'm having trouble finding out how to reduce these errors.  Can you help?

Thanks!