In and On in Emails: Which One Is Correct?

Quick Answer

Use “in” when you mean inside the email message, such as attachments, text, or links. Use “on” when you refer to a subject, topic, or a specific point like a date or a line item. Here is a quick reference:

• In the email = inside the message (e.g., “in the email below,” “in the attachment”)
• On the email = about the topic or subject line (e.g., “on the email thread,” “on the subject of”)
• On is also used for days and dates: “on Monday,” “on the 15th”

Understanding the Core Difference

The confusion between “in” and “on” in emails is common because both prepositions can appear in similar contexts. The key is to think about the relationship between the object and the email.

When to Use “In” in Emails

Use “in” when something is physically or conceptually inside the email message. This includes:

• Attachments: “I have attached the report in this email.”
• Text or content: “The instructions are written in the email.”
• Links: “You will find the link in the email I sent yesterday.”
• Quoted material: “As stated in the email from Sarah…”

When to Use “On” in Emails

Use “on” when you are referring to a topic, subject, or a specific point of reference. This includes:

• Subject lines: “I am writing on the subject of your invoice.”
• Topics: “Let me update you on the project status.”
• Email threads: “Please see my comments on the thread below.”
• Days and dates: “I will send the file on Friday.”

Comparison Table: In vs. On in Emails

Natural Examples

Here are real-life email sentences that show the correct usage:

• In: “Please find the signed contract in the attachment.”
• On: “I am writing to follow up on the proposal we discussed.”
• In: “The error was noted in the email from last week.”
• On: “Let me clarify on the budget line item.”
• In: “All the instructions are in the email below.”
• On: “I will send the update on Monday morning.”

Common Mistakes

Even advanced learners sometimes mix these up. Here are the most frequent errors and how to fix them:

• Mistake: “I have attached the file on this email.”
  Correction: “I have attached the file in this email.” (The file is inside the message.)
• Mistake: “Please see my comments in the subject line.”
  Correction: “Please see my comments on the subject line.” (You are commenting about the subject.)
• Mistake: “I will reply you in Friday.”
  Correction: “I will reply on Friday.” (Days use “on.”)
• Mistake: “As discussed on the email below…”
  Correction: “As discussed in the email below…” (The discussion is inside the email.)

Better Alternatives and When to Use Them

Sometimes you can avoid the confusion entirely by rephrasing. Here are some alternatives:

• Instead of “in the email,” try “in the message” or “in the attachment.”
• Instead of “on the email,” try “regarding the email” or “about the email.”
• For dates, you can also use “by” (e.g., “by Friday”) if you mean a deadline.
• For topics, “about” is often clearer: “I am writing about the project.”

When to use it: Use “in” for anything inside the email body or attachment. Use “on” for topics, subject lines, and dates. If you are unsure, rephrase the sentence to see if “inside” or “about” fits better.

Formal vs. Informal Tone

Both “in” and “on” are used in formal and informal emails, but the context matters:

• Formal: “Please find the details in the attached document.” (Correct and professional.)
• Informal: “Check the details in the email I sent.” (Still correct, but more casual.)
• Formal: “I am writing on the matter of your account.” (Polite and clear.)
• Informal: “Just following up on that thing we talked about.” (Acceptable in casual emails.)

In very formal writing, avoid using “on” for vague topics. Instead, be specific: “I am writing regarding your invoice” is better than “I am writing on your invoice.”

Mini Practice Section

Test your understanding with these four questions. Choose “in” or “on” for each blank.

1. Please find the agenda ______ the email I sent yesterday.
2. I will follow up ______ the sales report next week.
3. She included her phone number ______ the message.
4. Let me comment ______ the point you raised.

Answers:

1. in
2. on
3. in
4. on

Frequently Asked Questions

1. Is it “in the email below” or “on the email below”?

It is “in the email below.” The phrase refers to the content inside the email that appears lower in the message.

2. Can I use “on” for email attachments?

No. Attachments are inside the email, so use “in.” For example, “The file is in the attachment.”

3. What about “on the email thread”?

Yes, “on the email thread” is correct when you mean you are commenting about the thread. For example, “I have a note on the email thread.” But if you mean the text inside the thread, use “in.”

4. Is “in Monday” ever correct?

No. Use “on Monday” for days. “In” is used for months, years, and longer periods (e.g., “in March,” “in 2024”).

Final Tips for Real Writing

When you are drafting an email, pause for a second and ask yourself: Am I talking about something inside the message, or am I talking about a topic or date? If it is inside, choose “in.” If it is a topic or a day, choose “on.” With a little practice, this will become automatic. For more help with similar word differences, explore our Writing and Email Differences guides, or check out our Grammar Word Differences section for other common confusions. If you have questions, feel free to contact us or visit our FAQ page.