5.3.3 Unrecognized command with Exchange

Hello Yehudah,

Since a few weeks mailing to an Exchange server does not work anymore.
The message I get, when I test, is:
5.3.3 Unrecognized command
This is in the Exchange server log:

2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,6,172.27.172.1:587,172.27.172.5:47054,>,250-SIZE 36700160,
2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,7,172.27.172.1:587,172.27.172.5:47054,>,250-PIPELINING,
2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,8,172.27.172.1:587,172.27.172.5:47054,>,250-DSN,
2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,9,172.27.172.1:587,172.27.172.5:47054,>,250-ENHANCEDSTATUSCODES,
2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,10,172.27.172.1:587,172.27.172.5:47054,>,250-AUTH GSSAPI NTLM,
2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,11,172.27.172.1:587,172.27.172.5:47054,>,250-8BITMIME,
2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,12,172.27.172.1:587,172.27.172.5:47054,>,250-BINARYMIME,
2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,13,172.27.172.1:587,172.27.172.5:47054,>,250 CHUNKING,
2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,14,172.27.172.1:587,172.27.172.5:47054,<,STARTTLS,
2021-03-15T15:56:51.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,15,172.27.172.1:587,172.27.172.5:47054,*,Tarpit for '0.00:00:05' due to '500 5.3.3 Unrecognized command',
2021-03-15T15:56:56.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,16,172.27.172.1:587,172.27.172.5:47054,>,500 5.3.3 Unrecognized command,
2021-03-15T15:56:56.774Z,ORTELIUS\Client Frontend ORTELIUS,08D8DF5EF660492C,17,172.27.172.1:587,172.27.172.5:47054,-,,Remote(SocketError)

What could be the problem?
Thanks!

With kind regards,

Jac