How to Do a Mail Merge in Outlook: Classic and New Outlook

A mail merge in Outlook starts in Word and sends through classic Outlook for Windows. Word cannot pass its merged emails to new Outlook, so while new Outlook is your email app, Word tries to open classic Outlook to send them.

A small accountancy practice in Exeter emails its 240 self-assessment clients every January: "Hi «First_name», we still need your «Missing_item» before the deadline." This year the practice moved to new Outlook, and the merge stopped.

Why isn't mail merge working in new Outlook?

Word's email merge works only with classic Outlook. Word hands each finished email to a desktop mail program through MAPI, a Windows standard for passing email between programs. New Outlook does not support it (Microsoft Q&A, November 2025).

The fix is to run the merge from classic Outlook. Switch off the New Outlook toggle in the top-right corner, or open Outlook (classic) from the Start menu.

Both versions can run on one computer, and Microsoft will support existing classic Outlook installations until at least 2029 (Microsoft Learn).

If classic Outlook is not on your computer, check your plan: Business Basic includes only the web and mobile versions of Word and Outlook, and the merge needs both desktop apps.

How to do a mail merge in Outlook with Word

A mail merge from Excel to Outlook uses three programs: Excel holds the list, Word holds the message and classic Outlook sends it. We checked every menu name below against Microsoft's support pages and its mail merge quick reference guide in October 2026.

Prepare the spreadsheet

Put column names such as First_name, Email and Missing_item in the first row, keep the list on the first sheet and save a copy on your computer. Phone numbers, dates and amounts need extra care before Word reads them; mail merge from Excel shows how to set up the sheet.

Build the email in Word

1. Open a blank document and go to Mailings > Start Mail Merge > E-mail Messages.
2. Choose Select Recipients > Use an Existing List, then choose the file and the sheet. Choose from Outlook Contacts is the other option if your list lives in classic Outlook.
3. Click Edit Recipient List and untick anyone this email is not meant for.
4. Write the message. Put the cursor where a detail belongs and choose Insert Merge Field, or use Greeting Line for a ready-made salutation.
5. Click Preview Results and step through a few clients with the arrows. Check for Errors runs a trial merge and reports fields Word cannot fill.

Send it from classic Outlook

1. Choose Finish & Merge > Send E-mail Messages.
2. In To, pick the email column. Type the subject in Subject line; it takes plain text, so every client gets the same subject.
3. Leave Mail format on HTML, which keeps your formatting, and Send records on All. To send part of the list, enter a range in From and To.
4. Click OK, and Word passes one email per row to classic Outlook. Watch the Outbox empty into Sent Items.

Save the Word document when you finish. It keeps its link to the spreadsheet, so next January the practice updates the sheet and runs the same merge again (Microsoft Support).

On a Mac, the last step is Merge to E-Mail, then Mail Merge To Outbox. Microsoft notes that this option stays unavailable until you set Outlook as the default email program.

When an Outlook mail merge is not working

Most failures come from the Outlook version, the spreadsheet or the sending account. Match what you see with the likely cause.

What you see	Likely cause	What to do
Word tries to open classic Outlook	New Outlook is your email app	Switch to classic Outlook and run Finish & Merge again
Word cannot read the list, or columns are missing	The file is open online only, or the data is not on the first sheet	Save a copy on your computer and move the list to the first sheet
Amounts or percentages look wrong	Excel's number format	Format that column as text
You cannot find a Mail Merge button in Outlook	Classic and new Outlook put it in different places	In classic Outlook it sits under People > Home; in new Outlook, under the arrow beside Send
Emails leave from your own address, not a shared mailbox	The send dialog has no From box	See the note on shared mailboxes below

Shared mailboxes cause the most frustration. Word sends from the default account of the Outlook profile that is open.

In Microsoft's community forum, people get round this by making the shared mailbox that account, or by giving it its own Outlook profile.

What new Outlook's Start mail merge does

Start mail merge, the option built into new Outlook, sends identical copies, not personalised ones. To use it, open a new message and select the arrow next to Send.

Each recipient gets a separate copy of the same email, and Microsoft states that it inserts no personal fields from your contacts (Microsoft Support). That suits a notice where every word is the same, such as an office closure.

	Word with classic Outlook	New Outlook's Start mail merge
Where you start	Word: Mailings > Start Mail Merge	A new message: the arrow next to Send
Recipient list	An Excel sheet, Outlook contacts or a list typed in Word	The people you add to the message
Personal fields such as a first name	Yes, any column in the list	No
Attachments	No, links only	Whatever you attach goes to everyone
Subject line	The same for everyone	The same for everyone

The practice's reminder names a different missing item for each client, so it still needs Word.

A personalised version is planned. Microsoft's roadmap lists Mail Merge (Advanced) as in development. Its September 2026 target has passed, and on 25 October 2026 Microsoft's help pages still described only Start mail merge.

Mail merge with attachments in Outlook

Word's mail merge cannot attach a file. Microsoft's instructions say you cannot add attachments to merged emails, but you can include links (Microsoft Support).

Word for Mac has a "send as attachment" format, but the attachment is the merged document itself, and the email body stays empty.

For a file that goes to everyone, a link does the job. The practice puts its document checklist on its website and links to it in the email.

For a different file per person, add a column with each file's OneDrive sharing link and insert it as a merge field. Share each file with that client only, so a forwarded email does not open it for anyone else.

Add-ins from the Microsoft marketplace can attach a separate file to each email. An add-in that sends through your own mailbox is still bound by the limits below.

In MailGraf, a file everyone needs travels as a button rather than an attachment. Set the button's link type to File, then pick the file, up to 10 MB, with Choose a file from media.

The click report lists that button under the label you type in Name in reports (button links help article).

Microsoft 365 sending limits for a mail merge in Outlook

A Microsoft 365 mailbox can send to 10,000 recipients in 24 hours and 30 messages a minute, and a Word merge counts against both.

The figures come from Microsoft's Exchange Online limits page, last updated 19 October 2026, unless a row names another source.

Limit	Figure	What it means for a merge
Recipients per mailbox	10,000 in any 24 hours	Once reached, nothing more leaves until earlier sends drop out of the 24-hour window
Sending speed	30 messages a minute	Extra messages wait for the next minutes; the practice's 240 emails take about eight minutes
Sending from a mailbox you have permission for	Counted against you, the sender	A merge sent as the shared mailbox uses your own 10,000
An address ending in onmicrosoft.com	100 external recipients a day for the whole organisation	A business that never added its own domain stops after 100 clients
External recipients for the whole organisation	12,006 a day with 10 licences (Microsoft Exchange Team, updated August 2026)	Colleagues merging on the same day share this allowance
Outlook.com for Microsoft 365 subscribers	5,000 recipients a day, of which 1,000 can be people you have never emailed (Outlook.com limits)	Free Outlook.com accounts get lower limits

The same Exchange page warns that Exchange Online "isn't suited to accommodate bulk-mailing scenarios". Its limits exist to slow down unwanted bulk email, and a newsletter to a large list runs into them first.

When a mail merge in Outlook is the wrong tool

A mail merge in Outlook suits a yearly note to current clients, not a monthly newsletter that sells. The practice's January reminder is the first kind: 240 clients, each with details they need.

The practice's monthly tax tips newsletter goes to 1,600 clients and prospects and sells its services, so it is marketing. Word adds no unsubscribe link and keeps no record of what happens after the send.

Those gaps, and the opt-out rules in the UK, are covered in what mail merge is and where it stops working.

Sent from the practice's everyday domain, the newsletter would also put that domain's sender reputation at risk.

From a Word merge to MailGraf

The client sheet comes in through a file import, and each mapped column can go into an email as a personalisation tag. The subject field takes tags too, through Insert personalisation, where Word's Subject line box accepts plain text only.

Microsoft's 10,000 a day and 30 a minute limit the practice's mailbox only. The monthly plan has no daily cap, so the 1,600-client newsletter leaves as one campaign within the monthly allowance. Afterwards, its report records who opened, clicked or unsubscribed, which Sent Items never could.

Keep Word for the January reminder. The tax tips newsletter moves to MailGraf through sign-up on the pricing page, with the client sheet as its list.