How can personalization break deliverability (merge errors)?

Merge errors occur when personalization tokens fail to populate correctly, leaving broken placeholders visible in your email-like \"Hi {{first_name}}\" or worse, \"Hi ,\" with a dangling comma. These failures create multiple deliverability problems: they look unprofessional, trigger spam complaints from annoyed recipients, and can even trip spam filters that flag malformed content as suspicious.

The root causes are usually missing data (the subscriber's first name field is blank), syntax errors (a typo in the merge tag), or encoding issues (special characters in names breaking the template). Each ESP has its own merge tag syntax, and a misplaced bracket or incorrect field reference will cause the token to render literally instead of being replaced.

Prevention requires two safeguards: always configure fallback values (so \"Hi {{first_name|default:there}}\" becomes \"Hi there\" when data is missing) and test extensively with edge cases before sending. Create test contacts with blank fields, special characters, and extremely long values to verify your template handles them gracefully. A broken merge tag doesn't just look amateur. It usignals to both recipients and filters that something is wrong with your operation.