One scenario where this behavior is observed, it's when the Activation Server is undergoing maintenance.

Whenever you try to activate, by providing your email address, usually the Activation Server replies immediately (or almost). But in the event that the Server is stopped ( for a scheduled maintenance, for example ), for obvious reasons, the Activation Confirmation Email is not sent to the user.

If the process was still saved (this might depend on the type of maintenance being done), usually when the Server is back On, it goes through the saved processes, and then sends the Activation Confirmation Email.

In the event that you made several attempts to Activate, and now received several Activation Confirmation Emails, Use the Link provided in the last email you received to confirm your product activation.

On the other end, if you didn't received any Activation Confirmation Email after reasonable time, assume that the Activation process has failed, and try again. Don't forget to check the Spam folder of your mailbox: Sometimes the Activation Confirmation Email is filtered (false positive). If the issue persists, try using a different email address for the process.

For more information about the Activation Process, other scenarios and troubleshooting tips, take a look at the following Blog Posts:

https://swym.3ds.com/#post:2775 - Activation and licensing models incl. troubleshooting tips

https://swym.3ds.com/#post:1291 - Installation and Activation Guide