A receiver-SMTP SHOULD send only the reply codes listed in section 4.2.2 of RFC-821 or in this document. A receiver-SMTP SHOULD use the text shown in examples in RFC-821 whenever appropriate.
A sender-SMTP MUST determine its actions only by the reply code, not by the text (except for 251 and 551 replies); any text, including no text at all, must be acceptable. The space (blank) following the reply code is considered part of the text. Whenever possible, a sender-SMTP SHOULD test only the first digit of the reply code, as specified in Appendix E of RFC-821.
Interoperability problems have arisen with SMTP systems using reply codes that are not listed explicitly in RFC- 821 Section 4.3 but are legal according to the theory of reply codes explained in Appendix E.