Message is missing in my e-mail folders -

Hello Astridh,

Be sure to try checking on the webmail version of your email, via logging in to the provider's site, to see if the same emails are accessible there. Depending on the configuration that can help to see if the issue is the provider's end or if they're not syncing as expected.

Some further steps for issues with receiving emails may be found here: If you can’t receive emails in Mail on Mac - Apple Support

• Check whether Mail is retrieving a large message or a message with large attachments. See View Mail activity.

• Check if you’re using filters in the message list that are preventing expected messages from being shown.

• Try sorting messages in the message list by Date or another attribute, or searching for messages, to verify if you received them.

• Check whether your account is offline or disabled (inactive).

• If you’re missing emails from a specific person or group, check whether you blocked messages from them.

• Try quitting Mail, then opening it again.

• Choose Window > Connection Doctor, then follow any instructions in the Details column. Check for problems logging in to email accounts or with the incoming Mail server, or with firewall software blocking network traffic.

• Check whether you’re affected by another firewall, such as software installed separately from macOS or built into an internet sharing router, or administered by your email account provider. Contact the appropriate person for information.

• If you’re still having problems, contact your email account provider to verify your user name, password, incoming mail server, and other required information. Then verify the information is entered correctly in Mail account settings.

If you’re logged in to a network using VPN, also make sure that you entered your VPN password correctly.

Sometimes it can also be beneficial to give things a quick refresh by quitting the Mail app (Mail menu > Quit Mail) and relaunching it. Similarly, restarting the Mac may also be helpful in some instances: Shut down or restart your Mac - Apple Support

Cheers.