If you have sent an email with attachments to an email address, then it is sent and they already have the attachments. It is likely they received that email within seconds of you sending it, but that does not mean they have already opened it. They may choose to save the attachment to their computer of which you have no access. Basically you cannot stop someone from opening an attachment that was sent to them in an email. If you sent a link to a location where you uploaded a file then you would need to remove the file from where ever you uploaded it to, then the link they have in email will not work - that's assuming they have not already accessed it. If they are on the same intranet (not internet) and you sent a link to a file on your computer and that file is in a shared folder which they have access then get the file out of the shared folder. But, as it is possible they already had the email within seconds of sending it, then it is possible they already have seen the file.