What's Causing This Error
The SMTP 450 error occurs whenever there is an issue with the DNS routing on the recipient's SMTP server. For example, you may run into this error if:
- The recipient server cannot find the email address on its server.
- The recipient mail server rejected your request as your email content did not pass through its filter.
- Your mail server's IP address gets denylisted.
Solution - Here's How To Resolve It
Here are some actions that you can take to solve the problem.
- Ensure that the email address you are trying to send an email to exists.
- Make sure that your email content does not simulate a spam email. For example, ensure that your email content does not try to sell a product or advertise about winning a prize. Many spam filters will reject emails with content on this spectrum.
- If your email contains attachments, make sure that they do not exceed the maximum size accepted by the recipient SMTP server. (some recipients specify a maximum attachment size of around 5MB).
- Use MXToolbox to ensure that your mail server's IP address is not denylisted.