Email services use a technique known as Sender Address Verification to validate email addresses, controlling the spread of spam messages.
When you send an email, the receiving email service (for example, Gmail) checks that your email address actually exists. If, for some reason, your email server fails to report that your mailbox exists, the receiving email service will reject your email.
Look for an error that states “550 Sender verify failed”.
If you receive this error, please contact HostPapa Support so that we can investigate the issue.