A valid email address can bounce for several reasons unrelated to its validity. Common causes include:
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The recipient's mail server might be down or unreachable (Temporary server issues)
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The recipient's mailbox is full and cannot accept new messages
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The message might exceed size limits set by the recipient's mail server
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The message might be flagged as spam or rejected by the recipient's server due to various factors, such as a blacklisted IP address, spam-triggering keywords in your email content, or sending a high volume of emails in a short period without proper warm-up, SPF/DMARK/DKIM DNS configurations...etc.
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Incorrect configuration
If the email is truly valid, the bounce is typically a soft bounce, and the issue may resolve over time.