This typically occurs when there is a sharing violation and files are locked either by smartermail or iis.

Review the mail clients settings to see if its possible to change the frequency in which the mail client communicates with the mail server, be it sending, checking mail etc.

You may want to also re-index their account. I have seen this happen before and a re-index sometimes solves it.

I have tested changing the frequency in an outlook client and also I have re-index the user but no success.

I don't know how to check "sharing violation and files are locked either by smartermail or iis."

but if I stop smartermail service and web server then restart again the client start receive email on their outlook and webmail but after some time it gets stuck again.

Mahesh, since SM 15x is no longer supported, have you thought about upgrading to SM 17x? If you do that and it does not resolve the issue, SmarterTools can help you to figure out what is going on since 17x is fully supported.

Hi Mahesh,

This link might be useful to you:

https://portal.smartertools.com/kb/a3017/unexpected-sync-behavior-with-outlook-and-imap.aspx 

If the usage is an issue, when you are clicking on Inbox folder, have you checked the performance by logging to your server? It might possible that when you are opening your Mailbox, it connects to server and getting stuck due to CPU/RAM.

HI Pedro,

Thanks for the reply.

I already confirmed [UIDVALIDITY 1] UIDs valid. 

The server currently using 4% Of CPU for a few seconds it goes to max 12%

Server Ram Current Usage 2.6 GB and normally it max 4 GB

HDD Usage 10 MB/Sec DIsk I/O

server config as follow

CPU

4 Core

Intel(R) Xeon(R) CPU E3-1231 v3 @ 3.40GHz

16 GB RAM

2 TB HDD 90% Full (100 GB Free Only)

I found today when the client closes his outlook the webmail works fine. also, I found the smartermail delivery log it says 

Add a message to mailbox call failed.  Reason:
Delivery attempt to user@domain.com to folder 'Inbox' failed.  Mailbox is locked.

"Mailbox is locked." by smartermail article this issue caused by "mailbox that is currently being accessed via POP" or in this case "IMAP"

also, the client is using Microsoft office outlook 2011 on Mac. 

Hello Mahesh, to check for sharing violations you'll want to use a program like Process Monitor from Microsoft/Sysinternals. This program can be configured to monitor the users' directories on the server hard drive for any access outside of MailService.exe. If you see other applications accessing the files/folders inside of the SmarterMail domain directories; this is indicative of a sharing violation.

The most likely culprit in those cases is a third party antivirus system, backup system, etc. Any access to the SmarterMail domain files outside of our process can cause issues loading/accessing mail content for end users.