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Afaik: due to messages with very long headers, there is nothing bad about this other than messing with our logs. You can assume these messages are spam, I've verified by looking at one.
This could theoretically be used to exploit a security issue called stack clash. There are no published exploits for remote code execution, but it's been posited that it is theoretically possible. If this is happening several times per minute, look into stopping it for security sake. Of course, an exim upgrade would certainly fix it.
Reference: https://lists.gt.net/exim/users/91714?page=last