alt Hacker NewsRFC822 explicitly says it is for readability on systems with simple display software. Given that the protocol is from 1982 and systems back then had between 4 and 16kb RAM in total it might have made sense to give the lower end thin client systems of the day something preprocessed.
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sumtechguy • today at 1:03 PM
Also it is an easy way to stop a denial of service attack. If you let an infinite amount in that field. I can remotely overflow your system memory. The mail system can just error out and hang up on the person trying the attack instead of crashing out.
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You could expect a lot more (512kB, 1MB, 2MB) in an internet-connected machine running Unix or VMS.