How do I investigate SMTP rejection errors?

Updated Nov 2, 2026

An email can be refused in two ways: immediately while sending, or later with a bounce report.

Where to look

  • Refused while sending: recorded as a failed send in MailRush; API requests return an error.
  • Bounce reports: grouped by reason on the Feedback Loop page, with the codes under Codes.

Reading the codes

  • Codes starting with 5 (such as 550) are permanent: do not retry the address without fixing the cause.
  • Codes starting with 4 (such as 421) are temporary: the server asks you to try later, often because you send too fast or it is unsure about your reputation.
  • Frequent temporary rejections are a sign to slow down: lower the daily limit or increase the delay between sends.

More details in the FAQ

The FAQ is in the MailRush.io app under Support (sign-in required).

Related articles

Didn't find what you need?

Sign in to MailRush.io and open a support ticket, and our support team will get back to you.

Contact support