To be able to send large attachments from mobile devices via ActiveSync with Exchange, you will need to make the following manual modifications to the web.config file for Microsoft-Server-ActiveSync virtual directory inside “sync” folder on your Exchange CAS Servers
If you don’t make these configuration changes, you will get errors on mobile devices like:
Cannot Send Mail “Mailbox is Full”
Event ID 1008 (On Exchange CAS Server in Event Viewer under Application)
Message: An exception occurred and was handled by Exchange ActiveSync. This may have been caused by an outdated or corrupted Exchange ActiveSync device partnership. This can occur if a user tries to modify the same item from multiple computers. If this is the case, Exchange ActiveSync will re-create the partnership with the device. Items will be updated at the next synchronization.
If you look further down into the Event ID you will find Maximum Request Length Exceeded under the Exception Message.
Even if you change the maximum send/receive attachment sizes on all configurations via Exchange GUI’s there is a “maxRequestLength” setting in the web.config file under “C:\Program Files\Microsoft\Exchange Server\V14\ClientAccess\Sync” which is the “Microsoft-Server-ActiveSync” virtual directory folder
As per this article, you need to change the value for “maxRequestLength” from “10240” (10 MB) to “25600” (25 MB) if you want to be able to send email attachments via ActiveSync from mobile devices up to 25 MB
Calculation for sizes eg 10 MB (10 x 1024 = 10240)
We don’t know why Microsoft doesn’t automatically change this configuration when attachment sizes are updated via the GUI as you think it would be a global configuration change for all Exchange services eg send/receive connectors, mailbox and ActiveSync connections
Once you change this configuration in the web.config on all of your Exchange CAS Servers you will need to do an IISreset and you will then be able to send large attachments via ActiveSync from mobile devices
You have to also take into account that in forums it states that if you tried to send a 24 MB attachment it would most likely fail because “33% MIME conversion bloat added to the message after MIME conversion”
We don’t know if you update/upgrade Exchange CAS servers to a new rollup service pack, if it will overwrite this web.config setting and change it back to the default configuration