Question:

Is it possible to activate the option "valid sender address required"?

Answer:

The sender verification is usually off and isn't something that is normally available on the Hosted Cloud.

This toggle is available with the Local cloud but not for general usage with the hosted solution.

Furthermore, it is recommended to only disable sender verification as a last resort, with all other options exhausted. The sender verification is an important anti spam check and should be used for optimal message filtering.