Will it reach the inbox?

It goes out through your mailbox, not ours

Tellmerge asks Microsoft to send each message from your account, the same way pressing Send in Outlook does. There is no relay in the middle, no sending service, no shared pool of IP addresses you inherit somebody else's behaviour from.

That matters more than anything else on this page. Most delivery problems come from misalignment — mail claiming to be from your domain while being sent by a service your domain has never authorised. SPF, DKIM and DMARC exist to catch exactly that, and setting them up for a new sending service is where people usually go wrong. None of it applies here: Microsoft signs the mail as it always has, from the domain it always has, and your reputation is the one you have been building for years.

Two side effects worth knowing. A copy of everything lands in your Sent Items, because Exchange handled it. And replies come back to you, like any other message.

Each person gets their own message

Every recipient is sent one email addressed to them alone, with their own merged content and their own attachment. Nobody is in a hidden BCC pile of two hundred, and there is no "undisclosed-recipients" header. An invoice addressed to one person carrying that person's PDF looks like what it is — ordinary business correspondence.

No tracking pixels, no rewritten links

Tellmerge does not embed open-tracking images and does not wrap your links to count clicks. We have no plans to. Both are strong signals to a spam filter, and both mean quietly reporting on people who never agreed to it. Their absence helps your mail land, which is a pleasant thing about a decision made for other reasons.

The limits are Microsoft's, and they are generous

Your mailbox has a daily ceiling that Tellmerge neither sets nor can raise. Roughly:

• Microsoft 365 work or school: about 10,000 recipients a day, and around 30 messages a minute.
• Outlook.com or Hotmail: a few hundred a day, less on a new account.

Microsoft publishes and occasionally changes these, so treat them as the shape of the thing rather than exact numbers. Going over the per-minute rate does not lose mail: Outlook tells us to slow down, and we do — Tellmerge waits exactly as long as the server asks and then carries on, and tells you afterwards how many were throttled. A send of several hundred simply takes longer than a send of ten.

One case where mail will not go out at all

If your address ends in .onmicrosoft.com — the default domain a Microsoft 365 tenant gets before a real one is added — external mail may be refused before it leaves Microsoft, with a bounce reading:

550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP

That is Microsoft's own anti-abuse layer, not the recipient's. The .onmicrosoft.com domain is meant for setting a tenant up, not for sending; mail from it is unsigned, has no DMARC record, and gets routed through a restricted outbound pool. Nothing at our end or yours changes it.

The fix is to add your real domain to Microsoft 365 and send from that — which is what you want anyway, since your customers should see your name in the From line. Internal mail to colleagues in the same tenant is unaffected.

What actually gets senders into trouble

Three things, all of them about the list rather than the tool.

1. Sending to people who did not ask. A bought list, or a scraped one, draws complaints — and enough complaints can get your whole mailbox restricted by Microsoft, which stops all of your mail, not just your merges. Tellmerge is built for people you already have a relationship with: your customers, your staff, your members.
2. Stale addresses. Every bounce is a small mark against you. If a third of your list is five years old, clean it before you send. Tellmerge flags blank, malformed and duplicated addresses at import so you can fix the file first.
3. Writing like a campaign. Free, act now, unsubscribe footers, an image where the text should be. Filters are tuned for that, and it is not what this tool is for anyway.

Send one to yourself first

In the preview step there is a button that sends the message you are looking at to your own address — the real subject, the real merged text, the real attachment, only the recipient changed. It takes ten seconds and tells you what your own filter thinks, how it renders on your phone, and whether the attachment opens.

If it lands in your Junk folder, so will everyone else's. Better to find out before the other two hundred go out.