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by JdeBP 5 days ago
Yes there should, and the maximum size of what fits into a 512-octet DNS/UDP query packet, less header and EDNS0 and whatnot, is going to be that limit, ultimately. Because SMTP-based Internet MTSes that do not look up MX resource record sets in the DNS, and do not begin at DNS/UDP, do not currently exist. (-:

Even if they magically all switched to starting at DNS/TCP, that is still only 64KiB less header and EDNS0 and whatnot. And in that case one also has to allow space for sending the actual resource record set back in the response, DNS/TCP not having DNS/UDP's luxury of truncation, so it's not even as large as that.
1 comments
Joker_vD 4 days ago
I've actually looked at RFC 1034 ยง 3 you've mentioned in another comment, and it states

    To simplify implementations, the total number of octets that represent a
    domain name (i.e., the sum of all label octets and label lengths) is
    limited to 255.
Which, AIUI, corresponds to maximum of 253 ASCII characters (including dots) in the common "subdomain.domain.tld" string format. So "\.{1,100}@\.{1,253}" it is (because, again, it's probably very likely that I wouldn't want to interact in the first place with people who think that the local part of their e-mail having more than a hundred characters is reasonable).
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