It seems that Outlook in Office365 checks the links in incoming emails beforehand.

How should a website handle this for links that can be opened only once? By ignoring page requests from Microsoft's IP range? Which range?

Hi,

Have you setup the account in multiple devices like PC, MAC or phone? Make sure the emails haven't been checked on other devices.

If you use any anti-virus or anti-spam program that scans the emails in Outlook, please make sure they are not the cause by disabling the software temporarily.

Anyway, I haven't heard that Outlook client checks links in incoming emails before; if you mean OWA of Office 365, please post the question in Office 365 Community forum:

http://community.office365.com/en-us/f/default.aspx

Regards,

Melon Chen
TechNet Community Su