If you are using the SMTP provider to send emails and they're not delivering most of the time, the reason is usually because the sender email doesn't match with the SMTP email that you've configured here:
Once you set up your SMTP provider, If you are getting an error when you send a test email in the Conversation
You can click the ⚠️(red triangle) icon to view more details about the error in the conversation
For the error that we display in the conversation, we will fetch the error we got from the SMTP provider and display it. If you are sending from the SMTP-integrated email already, please open a support ticket with the SMTP provider so they can provide the delivery status on the email.
Learn more about Why Can't I use My Free Email Address As The SMTP?
Setting alias For Google Gsuite SMTP: