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We are currently using the SMTP service in IIS 6.0 to send mails. All incoming relay mails are monitored by another service which processes the relay mail and deletes/moves it. It works fine!

We have then installed a certificate to be able to enable incoming TLS transmission and added some remote domains for outgoing TLS encryption. The remote domains are only for the receivers we know have TLS enabled – all other uses the default (local) domain without TLS encryption enabled.

Just after enabling TLS encryption on the remote domains it worked fine but after a few hours the SMTP service starts locking the files for several hours so the monitoring service cannot delete/move the files. The files are locked for more than 4 hours (sometimes it exact 4 or 8 hours) and sometimes after a few days they are still locked (and we restart the SMTP service to unlock the files).

We have tried to remove the certificate to disable incoming TLS transmission but that didn’t work.

If we cannot solve this then we have to use install and another 3rd party SMTP service – does anyone know a good SMTP service which enables 2-way TLS?

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I found the problem with the SMTP service locking the files! The SMTP service was configured with the name 'default' for the default domain instead of the actual domain name. When changed to the correct domain name the relay mails was created in the "drop" folder instead of the "queue" folder - and now they weren't locked. Now the monitoring service can process and delete the files instantly without waiting for hours...

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