You can resend gifts in case your friend's email gift message was lost. If you are resending to a new (or corrected) email address you should be able to resend immediately. If you are attempting to resend it to the same email address the Steam system requires that you wait 24 hours to prevent the spamming of users. When you resend a gift the previously generated email will become invalid and the “accept” link within it will no longer work. Your friend must use the latest version of the email when accepting the gift.