@yabado,
Then be more explicit about the issue, since the error description is rather vague.
The error description implies and/or can imply that you and/or the client (i.e. your customer)
- use exchange server to connect (explaining why there would be some similarity with the mentioned KB, otherwise that reference would not make any sense)
- have a mail client that does not support TLS at all (explaining why the specific error is being reported in logs)
- have a certificates that have expired (also explaining some part of the reported error)
- use firewall rules that block specific ports, necessary for the TLS handshake (also causing a similar error)
and so on.
In short, without any detailed error description and/or log output, nobody can give a concrete solution.
By the way, it is easy to disable TLS (see KB article referred to, amongst others), but that implies that the whole postfix mail server is not using the TLS method.
The latter implies that your client (i.e. customer) should take the risks for alleviating security, since it is not good practice for you (as a party, very likely to serve other customers) to create security holes (for all clients/customers), just in order to have one customer mitigate it´s problems with connecting.
Note that new versions of Exchange server and/or new mail clients AND even fresh installs of old mail clients can result in the error notification disappearing.
Also note that a clean install of postfix server will also, at least in most cases, result in the error notification disappearing.
Maybe the above is (again) not helpful, but have a go at convincing your customer that you, as a service provider, should not compromise security, in order to have one client (and also a client that is not prepared or willing to use a more modern, secure mail client) connect to your postfix server.
Kind regards....
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