Unfortunately, there is no possibility to delay execution of an action within a Business Rule. As a solution, we recommend you to mark new users in your Business Rule triggering After Creating a User (e.g. by setting a custom Boolean attribute to true) and send the Welcome Email in a Scheduled Task. The task will check whether the user has the Boolean attribute set to true and whether they have a mailbox. If all the conditions are met, the Scheduled Task will send the Welcome Email and clear the Boolean attribute to avoid duplicate messages.
If this solution meets your needs, we will provide you with detailed instructions on how to create the Scheduled Task and update your Business Rule.