Log showing emails sent - but customers not getting any emails?

Please someone help me, i have spent thousands on my business and this is my last step.

Everything is automated but WHMCS is showing that emails are being set in the email log but nothing is being recieved by the client, ive checked everything that i can think of and it looks fine!

Are the emails actually sent from your mail server? Check the mail server queue.

What provider are you using? Some cloud companies, for example, block port 25 and don't allow emails to be sent out from their network/servers. In some, you can request this to be open manually for your server, and some others like Google don't allow emails to be sent out from their services, so you will have to use a third-party email provider.

WHMCS is showing that emails are being set in the email log

Then they are.

WHMCS won't show that those mails are 'sent' unless they actually, physically are sent.

nothing is being recieved by the client

Likely due to spam filtering rules, or RBL practices. Many hosts (especially gmail, outlook/hotmail, AOL, yahoo) are getting smarter about this .

If you're sending via php mail() don't

You should be sending via SMTP, always and forever.

Check the server's mail logs, see what they have to say about the recipient IP address.