Email rejection messages, defined by a three-digit code, reveal crucial insights about delivery failures yet are often oversimplified into broad categories by ESPs.
Many large mailbox providers and spam filter developers share detailed lists of rejection messages and their meanings, yet some email service providers obscure these details for reporting.
Specific rejection codes provide nuanced understanding of issues like inbox capacity and nonexistent email addresses, which are often misclassified as 'temp fails' by nameless ESPs.
The complexity of email rejection requires careful categorization; broad classifications like 'soft bounce' or 'probable spam' may overlook critical individual failures in message delivery.
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