It is difficult to troubleshoot your issue with the information provided, although there are a few things we can try. The first thing you will want to verify is that the domain you are trying to send an email from is whitelisted in the exchange servers trusted senders list. Additionally, if you are using Office365 and have MFA (multi-factor authentication) enabled, you may need to use an account that has MFA disabled as the service account will intermittently require additional action for MFA.
If the previous steps do not help, you will need to understand whether an error is being thrown on the Sitefinity side, or the SMTP server is blocking the email. You can begin by looking at the Sitefinity error log which can be found on the file system at the following location <Site>/App_Data/Sitefinity/Logs/Error.log. Check to see if there are any errors relevant to the SMTP settings or notification service. If there are relevant errors, they may give us additional information on what is causing the issue.
In the case that there are no errors, you will need to view the logs on the SMTP server to figure out what is happening to those test emails.
If this information is not sufficient, feel free to reach out to firstname.lastname@example.org for further assistance.