Why am I receiving the error: "421 4.4.2 Error: timeout exceeded?"

Question

Why am I receiving the error: "421 4.4.2 Error: timeout exceeded"

SMTP error from remote mail server after end of data: 421 4.4.2 testingmx.cptechs.net Error: timeout exceeded

Answer

These errors are most often returned from the recipient server. This error is typically only a temporary routing issue or filter taking place on the receiving mail server. Also, it is normal for 421 errors not to bounce back right away, because it is expected that these email messages will eventually finish or return with a full error code as to why the email could not complete.