No mails may be sent to business partner &

What causes this issue?

The <LS>Contact</> indicator (in the master record under <LS>Control
Data</>) for this business partner states that this partner is not to
be contacted.

System Response

The mail is not sent.

How to fix this error?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.