Using the Received field can lead to false positives when one uses multiple MTA's within the same domain.

The proper method is to add a custom header to the email and for incoming emails detect if that particular header is present (if so, then you have a loop condition).

Custom header names should start with 'X-' so as not to conflict with predefined names, e.g.

X-Loop: cormanaz@domain.tld
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