Email with "Drop Down (Single Select)

Hello!
I use "Drop Down (Single Select)". How can I make that an email is sent five days after the "Waiting" value is selected? However, if I select a different value during this time, no mail is sent.

Best way to send scheduled emails is to use a newsletter extension like AcyMailing. The only way to reliably do what you're wanting is a scheduler of some kind. You could use CB Auto Actions new Batching feature and an auto action to send the emails for example, but you'll need to schedule that auto action to be ran using a CRON task. The below blog post shows the new batching feature available in CB Auto Actions.

www.joomlapolis.com/blog/kyle/18891-cb-auto-actions-8-1-0

There is no need to send mail on a schedule. I need to send 1 message to the moderator after 5 days if a certain value is selected in the "Drop Down" field. Could you explain with an example?

You're wanting to send an email 5 days after the field has changed. That is a schedule. You'd first need a way to keep track of when the field changed so you have something to condition against. You'd then not want to send the email again of course so you'd need to keep track of who was already emailed once the email is sent.

There is no out of the box solution for what you're wanting outside of CBSubs Mailer, which is primarily meant to work with CB Paid Subscriptions. This means you need to implement your own solution. It's possible to implement your own solution using CB Auto Actions and its new Batching feature with a few auto actions. The alternative is to see if something like AcyMailing can be used to schedule an email to be sent 5 days later in which case you'd use a single auto action to schedule that right when the fields value changes.