Mail merge subject lines are a make-or-break email minefield.

Why? Most people think plopping someone’s first name into a subject line counts as personalization. (And maybe also throwing “quick question” in there as well.)

The problem is, that’s not enough anymore. At best, it’s lazy. At worst, it screams “mass email” — because, in the year 2025, no one is fooled anymore into thinking first name = one-on-one message.

Effective mail merge personalization goes several levels deeper. It shows you actually understand the person you’re emailing: their business, what’s causing them pain, their urgent needs.

In this guide, I’ll cover exactly what works (and what doesn’t) so you can write subject lines for your mail merge campaigns that get opened.

Mail Merge Subject Lines: Table of Contents

• What You Shouldn’t Include in Mail Merge Subject Lines
  • Just First Names
  • Generic Job Titles
  • Overly Complex Merge Fields
• What You Should Include in Mail Merge Subject Lines
  • Company Names (When Formatted Correctly)
  • Specific Personalized Metrics
  • Competitor Names
  • Industry-Specific Terminology
  • Location-Based Information
• The Best Tool for Mail Merge Subject Lines: GMass

What You Shouldn’t Include in Mail Merge Subject Lines

Let’s start with the bad stuff.

These are the mail merge subject line “personalizations” that actually hurt your open rates — and might even motivate someone to click “mark as spam.”

Just First Names

Here’s the truth: putting someone’s first name in a subject line doesn’t make you look personal. It makes you look like every other marketer who thought they were being clever in 2006.

Everyone knows you’re using mail merge when they see “Hey Sarah, quick question” in their subject line. It’s been overdone to death, and recipients have become completely numb to it.

Plus, and even more important, first names add zero actual value. They don’t tell the recipient why they should care about your email or what’s in it for them.

Examples of what NOT to do:

• {FirstName}?
• {FirstName}, quick question
• Quick favor, {FirstName}?

Generic Job Titles

Job titles in subject lines are almost always a bad idea because they’re either too broad to be meaningful or they’re flat-out wrong.

A lot of the job title data in the databases everyone uses is outdated, incomplete, or just plain inaccurate. Same deal if you’re using an AI tool to do prospect research.

Nothing kills credibility faster than addressing someone as “Marketing Manager” when they’ve been VP of Marketing for two years. Or when they’re actually a hiring manager or engineer.

Even when the title is correct, it doesn’t create any real connection or urgency. “Attention Marketing Manager” doesn’t tell someone how you’re going to help with their specific urgent pain point.

Examples of what NOT to do:

• For {JobTitle}s only: new strategy guide
• Hey {JobTitle}, quick question about your goals
• {JobTitle} insights for Q4 planning

Overly Complex Merge Fields

Some people get so excited about mail merge’s potential they try to cram everything into the subject line. The result is usually a mess.

Complex merge fields can break, display incorrectly, or just look weird.

Plus, long subject lines get cut off in most email clients anyway — and are proven across billions of emails to have lower open rates.

Keep it simple. One, maybe two merge fields maximum. Any more than that and you’re playing with fire.

Examples of what NOT to do:

• {FirstName} from {CompanyName}: {ProductName} results for {Industry} companies
• Re: {CompanyName}’s {Department} team and {CurrentPlatform} integration
• {FirstName}, {CompanyName}’s {JobTitle} position and our {ServiceType} solution

What You Should Include in Mail Merge Subject Lines

So if everything above is what you shouldn’t use — what should you use?

These are the mail merge elements that can improve your open rates because they show deep knowledge, address real needs, and foster genuine curiosity.

Company Names (When Formatted Correctly)

Company names can be incredibly effective in subject lines — but only if your data is clean and the context makes sense.

The key is making sure your company name appears natural. You don’t want “SEO audit for ACME CORPORATION INC” when “SEO audit for Acme” is what someone would type if this were a one-on-one pitch.

Clean your data first. Strip out “Inc,” “LLC,” “Corporation,” and other legal suffixes unless they’re actually part of how the company presents itself publicly. (You can use AI to handle data cleaning like this pretty efficiently.)

Examples that work:

• How {CompanyName} can cut costs by 30%
• {CompanyName}’s SEO audit – I found {ErrorNum} errors
• Benchmark report: {CompanyName} vs. {TopCompetitor}

Specific Personalized Metrics

This is where mail merge subject lines really shine. When you can reference actual numbers, percentages, or metrics specific to each recipient, you create immediate relevance.

Personalized metrics work because they’re impossible to ignore — the recipient knows you’ve done your homework. So even if they suspect this is a mass email blast, it’s one that’s fueled by real research and one-on-one attention.

The more specific the metric, the better. Generic numbers don’t work (saying you’ll improve someone’s business “10x” is so much less compelling than saying you’ll help grow their monthly traffic of 165,314 visitors by 42%).

Examples that work:

• Increase your {CurrentTraffic} monthly visitors by 42%
• Your {EmployeeCount}-person team’s retention strategy
• How to improve your {CVR}% conversion rate

Competitor Names

Mentioning a competitor in the subject line is psychological gold.

It’s triggering the fear center in someone’s brain if you do this right — which is a fantastic way to get their undivided attention.

The key is being subtle about it. You want to pique interest, not come across as gossipy or unprofessional. Frame it around insights, comparisons, or industry movements.

Examples that work:

• Why {CompetitorName}’s pricing strategy is working
• My 3 ideas for getting {CompetitorName}’s customers to switch
• Benchmarking against {CompetitorName}’s new strategy

Industry-Specific Terminology

Using the exact language your recipients use in their day-to-day work makes your email feel like it belongs in their inbox.

This is especially powerful in B2B campaigns where different industries have their own jargon, metrics, and pain points. It shows you understand their world, which serves as important proof that (1) you’re worth listening to and (2) you’re worth hiring to fix the problem.

But you have to be precise with the terminology, because they can spot a carpetbagger on sight.

Examples that work:

• Reducing your {SpecificMetric} by 25% this quarter (where SpecificMetric might be “CAC,” “churn rate,” or “COGS”)
• New {IndustryRegulation} compliance requirements
• Optimizing your {IndustryProcess} workflow (like “order fulfillment” for eCommerce or “patient intake” for healthcare)

Location-Based Information

Geographic personalization works particularly well for local businesses, regional campaigns, or when referencing location-specific events or regulations.

This creates immediate relevance because it shows you understand their local market conditions, challenges, or opportunities.

Location data is usually pretty reliable, making this a safe bet for personalization that won’t backfire.

Examples that work:

• Are you ready for {State}’s new tax regulations?
• {City} market expansion opportunities
• {LocalCompetitor} is dominating {City} — let’s change that

The Best Tool for Mail Merge Subject Lines: GMass

In case you didn’t know, Gmail has a built-in mail merge tool, which it offers on some workspace plans.

It’s fairly rudimentary. And one of its biggest flaws is: It explicitly doesn’t allow subject line personalization. You can merge fields in the email body, but the subject line stays static. That’s a massive limitation that kills the effectiveness of your campaigns before they even start.

Which is why hundreds of thousands of people (nearly 400k at last count) use GMass instead of Google’s own tool for their Gmail mail merges.

If you’re not familiar, GMass is a Chrome extension that works directly inside Gmail — and it lets you use mail merge in subject lines.

But GMass goes way beyond that. After all, subject line personalization should be table stakes. GMass also gives you:

• Advanced mail merge personalization options like conditional content, personalized images, personalized links, and personalized attachments — all via a Google Sheet of contacts.
• Scheduling tools to send your campaigns at the perfect time (including skipping weekends and holidays).
• Detailed analytics so you can see which subject lines are actually working. GMass is one of the few email platforms that even shows you open rates with bots removed.
• Automated follow-ups that keep your campaigns running until you get responses.
• Deliverability tools like Spam Solver, which tests your campaign on real mailboxes and helps you improve your inbox placement rate.

The best part? GMass slides seamlessly into Gmail, so sending a mass mail merge campaign isn’t much different than sending your day-to-day emails.

There’s no new platform to learn, no complicated setup process. You install the Chrome extension and start sending better mail merge campaigns immediately.

Ready to start writing subject lines that actually get opened and drive results? Just install the GMass extension and try it free — no credit card required.