Rarely emails are rejected, but today I got a bounce from AOL:

554 Message not accepted due to failed RFC compliance

What can be the reason for this? I read that some strangeness (Exotic To-field content, too many invalid/rare characters/data in the mail body) in the actual email can cause this.
How can I find out the underlying reason?
Blacklists come back negative so far for the mail server (except for one).

18 days later

I am French and I had the same problem with yahoo mail . Did you resolve this?

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    Michele Did you resolve this?

    Actually, no. I instructed the user to re-send that email. Apparently it worked because the user didn’t follow up with the same mailer daemon email.

    From what I read and understand this has something to do with IP address block to which the email server belongs.
    When there is lots of spam originating from that IP address block, the whole IP address block is temporarily blacklisted/limited.

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