I have started to see this happen more and more.  I have users that are getting the conflict message show when a user opens up email messages.  They have to keep the item or keep all.  What would be causing the conflict.  The users in question are using outlook in cached mode.  We are using exchange server 2010.  I have seen this happen in outlook 2010 and 2013.

Would the user approaching the mailbox limit cause the conflicts to happen?

Hi,

Please provide more details about this issue, such as, what is the error message?

If it's about the mailbox quota, it may be caused by a corrupted .ost file. I suggest you rename the .ost file, then restart Outlook, a new .ost file will be generated and check if the problem persists.

If it warns you the user is approaching themailbox limit, the error message may also prompt, consider to delete some unimportant items or archive some items to local.

Regards,

Melon Chen
TechNet Community Su

More data required.

Have a look at http://www.linkedin.com/today/post/article/20141111101047-198668846-resolve-the-unwanted-outlook-pst-errors-in-a-jiffy?trk=mp-reader-card for some common Outlook errors.