Hi,

We are using Exchange Server 2013.we have Outlook 2007 SP3 and 2010 SP2 client .Some clients inform mails are return.

Return massage

"This message could not be sent. Try sending the message again later, or contact your network administrator. Your server administrator has limited the number of items you can open simultaneously. Try closing messages you have opened or removing attachments and images from unsent messages you are composing. Error is [0x80040305-0x0004de-0x00054a]"

Thanks,

Gayan

Are there any events generated in the eventvwr?
like 

looks like this is a similar case. (link)

Maybe this will help.

1) Disable TCPChimney and RSS on CAS and Mailbox Servers by running below commands

netsh int tcp set global chimney=disabled

netsh int tcp set global rss=disabled

2) Disable unwanted Add-Ins in outlook that may open many sessions

3) If above steps doesnt resolve, you can delete existing outlook profile and create new Outlook online profile