Why your emails are going to spam (and how to fix it)
A quote goes out, nothing comes back, and you chalk it up to a customer gone quiet. Often the email's just sitting in their junk folder, and you can prove it and fix it yourself.
The short answer
First prove it's happening: send a real quote to a Gmail and an Outlook.com address you control and see whether it lands in junk. If it does, fix authentication first (SPF, DKIM and DMARC), then look your domain up on Spamhaus's checker at check.spamhaus.org in case you're on a blocklist. Stop BCC-ing customer lists from your ordinary mailbox; use a newsletter tool for that. Ask regular customers to add you to their contacts. And don't bother rewording subject lines first, because sender reputation beats wording every time.
Why good email ends up in junk
It's a horrible problem because it's invisible. There's no bounce, no error message, nothing. The customer thinks you ignored them, you think they ignored you, and a good job dies over a technicality.
Gmail and Outlook score every incoming message before deciding where to put it. The score comes from three things: whether your domain can prove the mail really came from you (authentication), how your address and the server behind it have behaved lately (reputation), and only distantly, what the message actually says. Most small firms fixate on the third and never look at the first two.
And it's usually not your fault. Google tightened its sender rules in February 2024 and Outlook.com followed in May 2025, so a setup that worked fine for years can now look suspicious without anyone having changed a thing. It hits the important messages hardest too. Quotes and invoices carry PDFs and payment links, exactly the shape of message filters are paid to be nervous about.
1. Prove it's actually happening
Don't guess. Set up a free Gmail account and a free Outlook.com account if you don't already have them, then send yourself a real message: an actual quote with the PDF attached, written the way you'd write it to a customer. Check both inboxes, and check both junk folders. Do it from your main address, and separately from anything else that sends as your firm, such as your invoicing software.
Then get a second opinion from mail-tester.com. It shows you an address to send to; send the same email there and it runs a spam test on the message. Keep a note of the result, because you'll want to repeat the same test after each fix to see what moved.
https://www.mail-tester.com2. Fix authentication before anything else
If your test landed in junk, the first suspect is always authentication: the SPF, DKIM and DMARC records that prove your mail is really yours. Google has required SPF or DKIM from every sender since February 2024, and warns that unauthenticated messages might be marked as spam or rejected outright. Outlook.com began enforcing the same three standards for high-volume senders in May 2025.
I've written a separate walkthrough that covers the whole job, including how to check that all three pass on real messages: https://lodesa.co.uk/library/spf-dkim-dmarc-setup. Do that guide first, then repeat your Gmail and Outlook tests. For a fair number of firms that's the whole fix, and you can stop reading here.
3. Check whether you're on a blocklist
Filters also judge the company you keep. If your email comes bundled with cheap web hosting, your messages leave from a server shared with hundreds of strangers, and Google's guidance is blunt about it: the activity of any sender on a shared IP address affects the reputation of everyone on it. One spammer on your server and your invoices pay for it.
So look yourself up. Spamhaus, the biggest name in blocklists, runs a Reputation Checker where you can enter your domain name or your mail server's IP address. MXToolbox has a similar blacklist check if you want a second opinion.
If you're listed, read what the listing page says before panicking. A listing against a shared server's IP usually isn't about you at all. You can report it to your host, but only they can get it delisted, and it often comes straight back. In my experience the lasting fix is moving your mailboxes to Microsoft 365 or Google Workspace rather than staying on the mailbox that came free with the hosting; sending reputation is those companies' whole business.
https://check.spamhaus.org
https://mxtoolbox.com/blacklists.aspx4. Stop bulk-sending from your everyday mailbox
The classic mistake is BCC-ing your whole customer list a newsletter or a price-rise letter from Outlook. To a filter, one address suddenly sending an identical message to 200 people is exactly what spam looks like, and it only takes a couple of recipients pressing "report spam" to dent the reputation of the domain your quotes and invoices ride on. Google's guidance says user spam reports can lower your domain's reputation over time.
The big providers now publish hard rules for bulk mail. Gmail requires anyone sending 5,000 or more messages a day to Gmail accounts to have SPF, DKIM and DMARC, a one-click unsubscribe on marketing messages, and a spam rate kept below 0.3% in its Postmaster Tools. Outlook.com brought in the same authentication requirement for domains sending over 5,000 emails a day in May 2025, and sends non-compliant mail to the junk folder. You may never hit those numbers, but they tell you exactly what the filters are tuned to punish.
So keep the two kinds of mail apart. Send newsletters and offers through a proper newsletter service, which handles unsubscribes and sends from its own infrastructure, and let your invoicing software send invoices one at a time. If you do a lot of marketing, put it on its own subdomain so a bad campaign can't drag your ordinary mail down with it.
5. Don't blame the wording
There's a stubborn folk belief that words like "free" or "offer", or attaching a PDF, are what send mail to junk, and that rewriting the subject line will fix it. In my experience that's years out of date. Modern filters weight who's sending far more heavily than what's said; an authenticated sender with a clean reputation can put "FREE OFFER" in the subject and land in the inbox, while a flaky one gets junked saying nothing at all.
Attachments follow the same logic. An invoice PDF from a domain that passes SPF, DKIM and DMARC is routine. The same PDF from a domain that proves nothing looks like invoice fraud, because that's precisely how invoice fraud arrives. Basic hygiene still applies (don't write in all capitals, don't send an email that's one giant image, keep links pointing at your own domain), but if steps 1 to 4 check out, the content is rarely your problem.
6. Ask your regulars to safelist you
For the handful of customers whose filters still disagree, enlist the recipient. Ask them to add your sending address to their contacts, and if a message of yours is sitting in their junk folder, to open it there and mark it as not junk rather than just dragging it out. In my experience both actions teach their filter, and your future mail gets the benefit.
It also helps to get the first move from them. When you take on a new customer, reply to their enquiry rather than opening a fresh thread; mail that's part of an existing conversation has an easier ride than a cold first message. And put your sending address on your paperwork, so if invoices come from [email protected], the quote says so.
Where this gets hard
The case you can't fix from outside is another company's filter. Your tests pass, Gmail and Outlook are happy, and one customer's corporate mail system still quarantines you. Only their IT people can see why, and only they can release the message or add your domain to their allow list. All you can do is ask nicely, by phone.
Shared-IP listings on budget hosting are the other wall. The checkers will show you the listing, but you can't clean up the neighbour who caused it, and hosts are slow to care because it isn't their invoices going missing. The honest fix is usually moving your mail to Microsoft 365 or Google Workspace, and that's a real migration: MX records, every mailbox, every phone and device that sends as you, with authentication redone at the end.
And reputation itself recovers slowly. There's no button that un-marks you as suspicious; the providers just watch how your mail behaves and adjust. Working out which of the causes above applies to you means reading message headers and DMARC reports, and knowing when the data says it's genuinely fixed rather than just quiet for a week.
What you end up with
- Proof of where your mail actually lands, tested against real Gmail and Outlook accounts rather than assumed
- Authentication passing, with the record-level detail handled by the SPF, DKIM and DMARC guide
- A clean blocklist result, or a named listing and a plan for dealing with it
- Bulk sends split off from day-to-day mail, so one campaign can't drag your invoices into junk
Facts on this page last checked 14 July 2026, against support.google.com, sendersupport.olc.protection.outlook.com, spamhaus.org, mail-tester.com.
Related fixes
If you've worked through this and something's still landing in junk, I'm happy to take a look and find out why.