occassional error when sending email?

Hello - A user received the following error when trying to respond to an email in CRM:

https://www.dropbox.com/s/x085qsdu5hzhvzs/Photo%20Aug%2003%2C%201%2020%2012%20PM.png

I asked him to retry and he was able to send the email successfully.  Any idea why this error might have occurred?  Do you get reports from users about this type of error occurring in CRM occasionally?  If so then how often? Is there a recommended way to help prevent this error? I'm just wondering, when a user reports this issue, should I simply instruct him to retry and advise that this is a known issue 1% of the time when responding to emails through CRM?