Not all emails are received

See the articles https://support.apple.com/en-in/guide/icloud/mm6b1a5aee/icloud

https://support.apple.com/en-in/guide/icloud/mm6b1a3b04/icloud

If the sender is forwarding large size mail messages with attachments and there is shortage of space in your mail account that mails will not download ( the sender must compress the attachments / files attach to mail ) , also the correct mail address of your should be used by them .

It depends what mail services is used by the sender , they do send the mails but are stuck in their outgoing mailbox and not received by you , a fault can be there from their server end , they are using incorrect port settings also depends they use window computers / Mac .

This article is for sender if they do use Mac https://support.apple.com/en-in/guide/mac-help/mchlp2528/mac

also check in your iCloud.com did you created a rule for that particular mail sender and moving that incoming mail to trash / folder , you can disable the rule https://support.apple.com/en-in/guide/icloud/mm6b1a3f8a/1.0/icloud/1.0

Try -- If you can’t receive emails in Mail on Mac - Apple Support OR Troubleshoot email problems in Mail on Mac - Apple Support.