Can't find some sent e-mails

Can't find some sent e-mails

I have used Zoho Mail through Chrome and things were going fine last months. However, I've just looked for some e-mails I sent hours ago and I can't find most of them.

The strange thing is that some of them appear but some of them don't.

What can I do about it? I need to be sure if my clients received them.

Best Regards,
Marcus