SMTP Reply codes are used to determine the error in the message, SMTP(Simple Mail Transfer Protocol) is a standard protocol for sending emails, the client sends and the server responds, a response is with the 3 digit code which contains the meaning for the response with the codes, find the below SMTP reply codes

SMTP Reply codes

Code	Description
211	System status, or system help reply.
214	Help message.
220	Domain service ready.Ready to start TLS.
221	Domain service closing transmission channel.
250	OK, queuing for node node started. Requested mail action okay, completed.
251	OK, no messages waiting for node node. User not local, will forward to forwardpath.
252	OK, pending messages for node node started. Cannot VRFY user (e.g., info is not local), but will take message for this user and attempt delivery.
253	OK, messages pending messages for node node started.
354	Start mail input; end with ..
355	Octet-offset is the transaction offset.
421	Domain service not available, closing transmission channel.
432	A password transition is needed.
450	Requested mail action not taken: mailbox unavailable. ATRN request refused.
451	Requested action aborted: local error in processing. Unable to process ATRN request now
452	Requested action not taken: insufficient system storage.
453	You have no mail.
454	TLS not available due to temporary reason. Encryption required for requested authentication mechanism.
458	Unable to queue messages for node node.
459	Node node not allowed: reason.
500	Command not recognized: command. Syntax error.
501	Syntax error, no parameters allowed.
502	Command not implemented.
503	Bad sequence of commands.
504	Command parameter not implemented.
521	Machine does not accept mail.
530	Must issue a STARTTLS command first. Encryption required for requested authentication mechanism.
534	Authentication mechanism is too weak.
538	Encryption required for requested authentication mechanism.
550	Requested action not taken: mailbox unavailable.
551	User not local; please try forwardpath.
552	Requested mail action aborted: exceeded storage allocation.
553	Requested action not taken: mailbox name not allowed.
554	Transaction failed.

See Also

Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry – https://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml

IP subnet calculator – https://computercarriage.com/2021/06/03/simple-ip-calculator-subnet-using-korn-shell-script-v1-0/

Shell script basics – https://computercarriage.com/2020/12/06/shell-script-basics/

rfc5248 – https://datatracker.ietf.org/doc/html/rfc5248