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I doubt it but I wonder if OP is lying and didn't realize there would only be a hundred or two emails

Can you restate.

Most probably Outlook! Imho the worst e-mail client on earth with a massive base of absolutely blind fanboys in almost every company...

Which Outlook? There are several different email clients with that name.

Outlook 2003?

my outlook 365 (or whatever it's called this week) at work can barely handle moving 100 without locking up

Irrelevant to the topic.

For me, it is Thunderbird, but only because Windows Defender scans all incoming emails AFAIK.