Deliverability is a critical aspect of email marketing. A common issue is ending up in the spam folder. This can be caused by poor sender reputation, incorrect email authentication settings, or using spammy words in the subject line. Blacklisting is another issue where your IP or domain is listed as a spam source, severely affecting deliverability. Ensuring email authentication with protocols like SPF, DKIM, and DMARC can help mitigate these issues.