If you see the error 550 5.1.0 Invalid Sender Domain, it usually means your mailing domain is missing an MX record.
Why This Happens
When you send an email, the recipient’s server may check your domain’s MX record to verify that your address is real and not spam.
If the MX record is missing, some ISPs will reject your email with this error.
Not all ISPs enforce this check, but many use it as a spam prevention mechanism.
Rule of thumb: Your mailing address should always be able to receive mail, which is why MX checks exist.
How to Check Your MX Record
Go to MXToolbox.
Enter your email domain.
Run an MX Lookup.
Review the results to confirm if your MX record exists and is properly configured.
How to Fix the Issue
If your domain is missing an MX record, add it in your DNS settings.
After updating, allow up to 24 hours for DNS propagation.
Once propagation is complete, retry sending your campaign.
By ensuring your MX record is set correctly, you’ll prevent this error and improve your overall domain reputation with ISPs.