What are SMTP reply codes?

SMTP reply codes are standardized three-digit responses during mail transmission:

Structure:

First digit: Class of response (2=success, 3=continue, 4=temp fail, 5=perm fail). Second digit: Category of response. Third digit: Specific condition.

Major categories:

2xx Positive completion: Action completed successfully. 3xx Positive intermediate: Continue with more data. 4xx Transient negative: Temporary failure, retry later. 5xx Permanent negative: Fatal error, do not retry.

Common examples:

220: Server ready. 250: Action completed OK. 354: Start mail input. 421: Service not available. 550: User not found.

Reply codes are the SMTP conversation's language. Each number tells the sender exactly what happened and what to do next.