That interface is probably 640x480, so of course it’s truncating things.
And I’m sorry, you really can’t fathom why someone who gets a ton of email would want to see more of them in their inbox at the same time?
Well, you can have a higher screen resolution than 640x480, resize those columns, hide the ones you don't use...
Look at the Thunderbird 1 and 3 screenshots on that page
Overview, seeing threads with more than a handful of e-mails completely, selection, tagging, dragging, deleting, marking as spam, in general all kinds of bulk actions.
time to make an email client...
Also, which use-cases do you have where you need to see 20 emails at once?
[^1]: https://www.pixelbeat.org/docs/netscape_email/ns_4_email.jpg