Which email client provides what is depicted in todays xkcd? (xkcd.com)

Yes, but that only provides mail composition with commonly used phrases.

Is there something providing better reminders, based on the content? Basically, like in the comic.

Very often do I send a mail with a content of basically "sending something in the attachment" without actually attaching anything.

In fact, the recent GMail got a feature to push old unreplied mails to the top (advertised for instance in https://www.indiatoday.in/technology/news/story/gmail-redesi... ). I think Randall refers to this feature.