Mac Mail Not Receiving Emails, and Sent Messages from today have disappeared!

I use Mac Mail for my work email, so it was very inconvenient for me to discover today that people who are trying to send me emails are having them bounce back. So I've received no emails today.

When I checked my Sent folder, all my sent mail from today is not showing (but the sent mail from yesterday and previous is).... yet people have received my emails.

Any help would be greatly appreciated - I'm tearing my hair out here!

Thank you.

I had the same problem on my iPad recently. I never made any changes to it and nothing I tried to fix it worked. I winded up deleting the email account and then created the same one again. Works good now.