Exchange 2007 periodically stops sending emails with attachments

What is the NPP at that time ?
Any events seen in the App logs ?

- Rancy

Nothing shows in the Application or System event logs. The only error being reported in the Application log is Event 2424, Source 'Windows Sharepoint Services 3 Search', but this appears even when the server is working properly.

There are the following errors in the 'System' event log but again these happen even when Exchange is running fine:

Event ID 10009, Source DistributedCOM
DCOM was unable to communicate with the computer ADMIN-PC.domain.local using any of the configured protocols.

Event ID 4, Source Security-Kerberos
The Kerberos client received a KRB_AP_ERR_MODIFIED error from the server office-pc$. The target name used was RPCSS/ADMIN-PC.domain.local. This indicates that the target server failed to decrypt the ticket provided by the client. This can occur when the target server principal name (SPN) is registered on an account other than the account the target service is using. Please ensure that the target SPN is registered on, and only registered on, the account used by the server. This error can also happen when the target service is using a different password for the target service account than what the Kerberos Key Distribution Center (KDC) has for the target service account. Please ensure that the service on the server and the KDC are both updated to use the current password. If the server name is not fully qualified, and the target domain (domain.LOCAL) is different from the client domain (domain.LOCAL), check if there are identically named server accounts in these two domains, or use the fully-qualified name to identify the server.

I'm sorry, what does NPP mean?

Non Paged Pool (its seen in task manager)
http://wintelguide.wordpress.com/category/windows/page/3/

Check when you restart and if OWA stops you check and let me know whats the value

- Rancy

The restarting of the server hasn't helped this time - the error still persists. The NPP is 89 right now. It seems to be only affecting one user at the moment. When you send an email with an attachment, it goes into the Outbox, starts to send, then it loses connectivity with Exchange and reconnects. The email stays in the Outbox, no other error messages in Outlook. The user uses Exchange over HTTPS.

Does OWA\Webmail works fine ?

- Rancy

I have just logged on to OWA using the user's details and it logs on fine.

Interestingly, if I send emails with attachments from Outlook Web Access from a different Internet connection they send fine. However, If I try the same thing on their Internet connection it doesn't work (the 'Attach Files' dialog box freezes).