A merge tag is a placeholder in an email that gets replaced with a value from the subscriber's record at send time. Hi {FIRSTNAME}, becomes Hi Jane,. It is the simplest form of personalization and the one most likely to embarrass you, because the failure is visible to the recipient and invisible in your preview.
The syntax, and why it varies
There is no standard. Different platforms use different delimiters — curly braces, double curly braces, percent signs, square brackets — which is why merge tags never survive a migration intact.
The important part is not the delimiter but the fallback. Every platform supports a default value in some form, and using it is the difference between a professional email and "Hi ,". Whatever your syntax, find the fallback form and use it on every tag, without exception.
The four ways merge tags break
1. The field is empty. The most common by far. Ask for a first name on a signup form and a large share of people will skip it, so a greeting built on {FIRSTNAME} renders as Hi , for a meaningful slice of your list. Always set a fallback, and pick one that reads naturally — "there" works, "Valued Customer" reads like a form letter.
2. The data is dirty. People type anything into a form. Hi jane, Hi JANE SMITH, Hi Mr. Smith, Hi asdf, and Hi ., are all things that have reached real inboxes. Normalise capitalisation on import, and reject or blank obviously junk values rather than merging them.
3. The tag is wrong. A typo in the tag name means either the literal {FIRSTNMAE} appears in the email, or — worse on some platforms — nothing appears and the sentence is quietly broken. This is why you preview against real subscribers rather than reading the template.
4. The field means something else than you think. A field called company populated by an import might hold the company that referred them rather than the company they work for. Look at the actual values before you build an email on a field.
What to merge and what not to
Worth merging: first name, company, city where it changes the offer, last product purchased, plan or tier, renewal date, account balance or usage, referral code.
Not worth merging: last name on its own, which is either formal or odd. Job titles, which are usually wrong. Anything whose blank state is worse than not personalising at all.
The test is: would the sentence still work if you deleted the tag? If yes, the tag is decoration and you can afford for it to be missing. If no, you need a fallback that makes the sentence work, or you need to restructure it.
The uncanny valley
There is a real limit past which personalization stops feeling like service and starts feeling like surveillance. Referencing a purchase is fine. Referencing browsing behaviour in a chatty tone is not. "We noticed you were looking at the blue jacket for a while" reads as being watched, even though the underlying data is the same data that makes a recommendation block feel helpful.
The reliable distinction: personalise what you offer, not how you talk. A product recommendation based on behaviour is useful. A sentence that narrates the behaviour is uncomfortable.
Data hygiene that prevents the problem
Ask for less at signup. Every field you request lowers conversion and raises the share that are blank or junk. Email address alone converts best. Collect first name only if you will actually use it, and collect everything else later, from behaviour rather than from forms.
Normalise on import. Trim whitespace, fix capitalisation, blank out values shorter than two characters or containing digits where they should not.
Check coverage before you build. If only 40% of your list has a first name, an email whose structure depends on it is an email that reads oddly for the majority.
Preview as a real subscriber, including one with the field empty. Most platforms allow this; it is the single most effective check.
For anything more sophisticated than substitution — showing or hiding whole blocks — you want dynamic content rather than tags, and for what to personalise at all, email personalization examples.
Frequently asked questions
What happens if a merge tag has no value?
Without a fallback, most platforms render nothing, producing "Hi ," or a broken sentence. With a fallback set in the tag, the default value appears instead. Set one on every tag.
Do merge tags affect deliverability?
Not directly. Broken merges affect engagement, and engagement affects deliverability, so a campaign that greets thousands of people with "Hi ," will underperform and contribute to a slow decline in inbox placement.
Should I use first name personalization?
It slightly helps when the data is clean and slightly hurts when it is not, so it depends entirely on your field coverage. If most of your list has no first name, restructure the greeting instead of merging one.
Can I use merge tags in the subject line?
Yes, and the same fallback rule applies with more at stake, because a broken merge in a subject line is visible in the inbox before anyone opens anything.
