How can I find emails not showing in Mail?

good morning everyone. I'm using mail version 13.4 with 10.15.7 (it's true, I have to update it...) and a very strange thing has been happening to me for months. From a single sender, I don't see the emails on Mail but the emails are there because I see on "webmail". I tried in the settings, but there is nothing that can allow a different view. I don't even know what to ask the person who sends me the mail. At this point I ask those who know more. thanks

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