\.{1,100}@\.{1,100}
\.{1,100}@\.{1,100}
I've had sites correct me with an email address ending in ".fi" with "are you sure you don't mean ".fr"?
Not unreasonable if those sites are mostly serving a French audience.
Realistically, the length of the domain part is likely ultimately constrained by how large a domain name can fit into a DNS/UDP query packet (alongside EDNS0).
Just had to update this this week - a previous dev had used 2,4 and someone came through complaining with a six character domain suffix. Apparently 24 or so is the current limit for a real domain suffix.
Even that's not the true length limit of a label in the Domain Name System. (RFC 1034 § 3, for the curious.) So someone is likely going to be fixing that, years down the line. Then of course there's the fact, as called out earlier, that there can be more than 2 labels in a domain name.