Plusnet Email Not Sending After the Move to Greenby? How to Fix 'Unknown Sender'

Your Plusnet email has worked for years. Then, without you changing anything, every email you send is rejected with an error like “Unknown sender”, while new emails still arrive as normal.

If that started around the time Plusnet moved its email service to a company called Greenby, the cause is almost certainly the address you send from. This guide explains what has changed, how to fix it in a few minutes, and what else to check once your account has moved.

Plusnet is closing its own email service and moving customers’ email, web space and domain names to a new supplier, Greenby. Force9 and several other brands in the same family are moving too. Your email address stays the same, and you keep your existing emails.

Accounts are being moved in stages, and Greenby emails you when yours is done. From then on you manage your email at greenby.com, logging in with your main Plusnet mailbox username and password.

For most people nothing else changes. But Greenby’s system is stricter than Plusnet’s old one about which addresses you’re allowed to send from, and that is what catches people out.

The wording depends on where you’re sending from:

• In an email program such as Thunderbird or Outlook: “Sending of the message failed. The mail server responded: Unknown sender.”
• In webmail: “SMTP Error (550): Failed to set sender (5.7.1 Unknown sender).”

Both mean the same thing: the server that sends your email has refused your own address. It has nothing to do with the person you’re writing to, even though some email programs confusingly tell you to “check the message recipient”.

The giveaways are that it happens whoever you email, it happens on every device and in webmail, and you can still receive email perfectly well.

Other providers use the same code for different reasons; my 550 5.7.1 page explains them.

A Plusnet account comes with a main mailbox named after your account username, for example smith@smith.plus.com. It also has a “catch-all”: email sent to anything@smith.plus.com is delivered to that main mailbox.

Because of the catch-all, many people simply picked a friendlier address, such as jane@smith.plus.com, and started using it. It was never actually created on the account, but it worked:

• Receiving worked because the catch-all delivered it to the main mailbox.
• Sending worked because Plusnet’s old sending server didn’t check which address you were sending from.

Greenby’s sending server does check. It will only send email from an address that exists on your account as a mailbox or an alias (an extra address attached to a mailbox). An address that only ever worked through the catch-all is neither, so it’s rejected as an unknown sender.

The fix is to add the address you send from to your account, as an alias of the mailbox your email already arrives in. Nothing moves, and you keep the same inbox and password.

1. Log in at greenby.com with your main Plusnet mailbox username and password. If Greenby says your login isn’t active yet, your account hasn’t been moved: see the note below.
2. Open Email in the menu. You’ll see a list of the mailboxes on your account. If the address you send from isn’t one of them, you’ve found the problem.
3. Add the alias. Open the menu (the three dots) at the end of the row for the mailbox your email arrives in, choose the option to add an alias, and enter the address you send from.
4. Wait a few minutes, then send a test email to a friend or to another address of your own.

Two things to avoid:

• Don’t create it as a new mailbox. A new mailbox has its own empty inbox and its own password, so your email would end up split between two places.
• Don’t make the change in Plusnet’s old Member Centre if your account has already moved. Changes made there don’t carry across to Greenby.

If you can’t log in to Greenby yet, your account is still waiting to be moved, but the stricter sending rule may already apply. Until Greenby confirms your account is ready, you can send from your main mailbox address (the one named after your username), which is accepted. Replies to it arrive in the same inbox.

Some people used Plusnet’s servers to send email from an address at their own domain name, such as you@yourbusiness.co.uk, often with the domain registered somewhere else and its email forwarded to a Plusnet mailbox. Plusnet’s old outgoing server, relay.plus.net, allowed this. Since the move to Greenby it no longer does, unless the domain itself is hosted with Greenby.

At the time of writing (last checked 6 October 2026), people in this position report these errors:

• “550 5.7.1 Unknown sender”
• “sender address invalid”
• “451 4.3.2 Internal server error”

An alias won’t fix this one, because the address doesn’t belong to your Plusnet account. Which fix you need depends on where your domain’s email is really handled.

If another company hosts your domain’s email

If your domain has proper mailboxes somewhere else, such as Microsoft 365, Google Workspace or your web host, send through that company instead. In your email program, change the outgoing (SMTP) server, username and password to the ones that company gives you. Emails then leave from the servers your domain’s records already trust.

If your domain only forwards to your Plusnet mailbox

This is common when a domain was bought from a registrar such as GoDaddy or 123 Reg and set to forward everything to a plus.com address. Incoming email still arrives, but nothing at the domain is set up to send. You have three options.

1. Send from your plus.com address, with replies going to your own domain. In your email program, add your own-domain address as the Reply-To address on your Plusnet account. People see your plus.com address as the sender, but their replies come back to your own domain. It is free and takes a couple of minutes, but customers will notice the plus.com address.

2. Use a sending service for your domain. A separate email sending service can send as you@yourbusiness.co.uk on your behalf. You sign up, verify your domain, and put its outgoing server settings into your email program. For your emails to be trusted, two records must be added where your domain’s DNS is managed (usually the registrar):

• SPF. Add the value the service gives you, such as include: followed by its name, to your domain’s SPF record. A domain may only have one SPF record, so add to the existing one rather than creating a second. Two SPF records make both invalid.
• DKIM. Add the record or records the service gives you, usually CNAME records with names ending in ._domainkey. Copy the values exactly.

Then send a test email to a Gmail address and check its headers show spf=pass and dkim=pass with your own domain. My guide to reading email headers shows where to look, and my guide to SPF, DKIM and DMARC explains what the records do.

3. Set your domain’s email up properly. If you rely on the address for business, move the domain to a provider with real mailboxes, so sending and receiving are handled in one place and a change at your broadband provider can’t break it again. My guide to setting up professional business email explains the options.

Once your account has moved, you can read and send email in Greenby’s webmail. There’s one catch I’ve seen. If you go straight to the webmail login page and sign in with just your short username, you can read your email, but sending can fail with “SMTP Error (535): Authentication failed”.

There are two ways round it:

• Sign in to webmail with your full main email address as the username, for example smith@smith.plus.com.
• Or log in at greenby.com, open Email, and click View Webmail next to your mailbox.

Then check the name and address your emails go out with. In webmail, open Settings, then Identities, and make sure the name is yours and the email address is the one you want people to see. I’ve seen webmail fill this in with an address that isn’t real, so it’s worth a look before you send anything important.

Since the move, some people see this error when they press Send, in webmail or in their email program:

• “SMTP Error: [451] 4.3.0 Error: queue file write error”

Some also report a “421 Mail system error” straight after it.

This one isn’t caused by your settings or your password. A 451 code means a temporary fault: Greenby’s sending server accepted your login and your email, but couldn’t store the email to send it on. I’ve seen it on an account where the login, the alias and the settings were all correct, and the same email went through when it was sent a second time. At the time of writing (last checked 13 October 2026), other Plusnet users report the same error on Plusnet’s community forum, some of them on and off for weeks.

What to do:

1. Send it again. It usually goes through on the second or third try.
2. Make sure important emails have actually gone. When this error appears, the email isn’t sent and isn’t tried again automatically, so nobody receives it unless you send it again.
3. If it keeps happening, report it to Greenby. Give their support team the exact error, the date and time, and whether it happens in webmail, your email program or both. Only Greenby can fix their server.

Don’t change your password or settings because of this error: it won’t help, and you could end up with a second problem to solve. If the error says “Authentication failed” or “Invalid login or password” instead, that is a login problem: see the section above.

• Your spam folder. Greenby’s spam filter is new to your account, so look through the spam folder for genuine emails before you empty it.
• Duplicate folders. Your old Sent and Trash folders may appear as extra folders, some with names like INBOX.Sent. Your old emails are in those. New sent emails go into Greenby’s own Sent folder.
• Your phone and computer. Greenby says existing Plusnet email settings carry on working. If a device still won’t send after you’ve added the alias, check that its outgoing server is set to log in with a username and password, and try your full main email address as the username.
• Storage. The Email page in your Greenby account shows how much space you’ve used. If it’s nearly full, delete old emails with large attachments.
• The cost. At the time of writing, Plusnet says the service is free for two years for its current broadband customers, then £15 a year. For everyone else it’s free for 30 days after the move, then £15 a year.
• Emails landing in spam. Your emails now come from different servers, so the first few may be treated with suspicion. If someone says they haven’t heard from you, ask them to check their spam folder. My guide to why emails go to spam explains more.

For most people, adding the alias is the whole fix: a couple of minutes in your Greenby account and your email sends again. If it still won’t send, note the exact wording of the error, because “Unknown sender”, “Authentication failed” and “queue file write error” have different causes, as described above.

If you rely on a plus.com address for your business, this is also a good moment to think about moving to an address at your own domain name, so that a change at your broadband provider can never interrupt your email again. My guide to switching email provider without losing emails explains how to make the move safely.

If you have a Virgin Media address instead, see my guide to Virgin Media email moving to Junara.

Email Fix is an independent service and is not connected with Plusnet or Greenby.

For other bounce messages, see why emails bounce.

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