SMTP Error 421 4.7.0 means the receiving mail server has temporarily refused your message under its own sending policy \u2014 most often because of rate limits, a reputation issue, or an authentication problem, not because anything is permanently wrong with the recipient address. It\u2019s a soft bounce: the message stays queued and your server will retry automatically, usually succeeding within a few hours once the underlying issue clears.<\/p>\n\n\n\n
Quick answer: to resolve SMTP Error 421 4.7.0, verify that your SPF, DKIM, and DMARC records are correctly published and aligned, confirm you haven\u2019t exceeded your provider\u2019s sending limits, check that your IP or domain isn\u2019t listed on a blacklist, and clean your list to lower bounce rates. If the rejections keep recurring after those checks, the root cause is usually sender reputation \u2014 which is where an email warmup tool like Warmy.io becomes useful for rebuilding trust with ISPs. For a broader reference on SMTP error codes, Warmy’s SMTP error codes and messages guide<\/a> covers the full range of delivery failures and how to fix each one.<\/p>\n\n\n\n The sections below break down exactly what triggers this rejection and how to fix each cause, so you can stop it from escalating into a permanent block.<\/p>\n\n\n\n