Hi @selçuk tan,
Since our forum mainly provides technical support in English, in order to avoid misunderstanding, it is recommended that you post in English. Thank you for your understanding.
According to machine translation, your question is as follows: One of my company mail users can exchange e-mails in the internal environment, but cannot exchange e-mails outside. This applies to 1 user.
May I know what version of Exchange you are using? Please check the email non-delivery report (NDR) to locate the possible cause of the problem. You can upload relevant screenshots after covering user privacy so that we can help you better.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.