Some mails could not be received due to the fact the Postfix 2.8 and higher in Plesk uses only TLS encryption, when senders use other encryption methods. Manually updating main.cf directly on the server (as discribed in https://plesk.zendesk.com/hc/en-us/articles/213386889) is not always an option since the main Postfix configuration file may be overwritten by any Plesk update. In order to avoid potential issues with non-delivered mails it is required to make SSL/TLS encryption on port 587 optional, not mandatory.
Thank you for your input! We will consider this functionality in upcoming releases if it will be popular.
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