topic Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i in Email

Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby interface

balvack2 — Mon, 16 Feb 2026 16:30:42 GMT

I've been logging into webmail regularly since being migrated to Greenby in December 2025 to retrieve the not-spam emails (including emails from Plusnet!) in my Junk folder.

This week I've tried for the first time to send email from webmail only to find I can't send, or reply to emails.

I have an 'identity' set up to be name@mydomain.co.uk instead of name@plusnetaccount.plus.com

This has worked fine for the best part of a decade, but now I get the following error if I try to send or reply:

SMTP Error (550):Failed to set sender “name@mydomain.co.uk" (5.7.1. unknown sender)

Since migration I've continued using the webmail.plus.net interface with server=Plusnet as advised.

I've now discovered there is webmail.greenby.com site. The identity has been migrated, but when I try to send/reply from that 'roundcube' webmail I get the same error. At least on the Greenby version I can select an alternative sender i.e. name@plusnetaccount.plus.com with Reply-to as name@mydomain.co.uk but that defeats the object of having an identity that hides the actual mailbox name.

Please, can someone please fix this at the server end? I am dependent on access to webmail when away from home and am scheduled to be away shortly....

(I've not raised this as a ticket on the Greenby site as I have absoluetely no confidence in their 'support' as I still haven't even received a reply to the one I raised on the 11th February about the major email outage that day)

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby in...

Leanne_T — Tue, 17 Feb 2026 09:49:56 GMT

Hi there @balvack2

Thanks for coming back to the community.

If you're getting this error message, I'd recommend running a virus/malware scan on your device and updating your password.

Once you have done this and if you still have the same error, please send me a private message with a screenshot of the error message/non-delivery report and the from email address and we will help you further.

Speak to you soon.

Leanne.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

balvack2 — Tue, 17 Feb 2026 13:32:12 GMT

Thanks for picking this up @Leanne_T

With all due respect, running a virus/malware check & changing passwords will not resolve a problem with the mail server:

I get the same error on 3 different devices, using 2 different browsers, with 2 different mailboxes that have "identities" configured, using both Plusnet and Greenby webmail interfaces. I'm not getting non-delivery reports as webmail won't even send the email. I get the same error when replying to an email.

I'll send you a PM shortly with screenshots of one of the affected accounts.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

mavison — Tue, 17 Feb 2026 15:03:39 GMT

@Leanne_T

I agree totally with @balvack2 comment
With all due respect, running a virus/malware check & changing passwords will not resolve a problem with the mail server:

I posted about this Unknown Sender problem a week ago.

Greenby support have told me that to send emails you must:

Either use the primary mailbox directly,
Or create an alias for any additional address you want to send from.

When I asked for clarification of the term Primary Mailbox I was confused about their definition:

Just to clarify, when you add an alias to a mailbox, it will function under that primary mailbox for sending and receiving emails,

It seems that to send an email there must be an alias for the sending address in the mailbox that is being used - even if it is a catch-all mailbox which can receive many receiver addresses. This certainly was not the case with Plusnet/Force9. They claim that this has been the case since migration (in December for me), but I have proof that it was a change made sometime between 19th January and 6th February, which they seem to ignore.

They also have stated that to send an email from a mailbox, you must be authenticated (with relay.force9.net in my case) with that mailbox name. This I do not believe to be the case - I always authenticate with my account name to send all my emails.

I now take what Greenby support (and the bot) with a large pinch of salt, as this undocumented change is causing me significant problems, as I use lots of names before my @account.f9.co.uk

Greenby claim that this is
to prevent spoofing and ensure that outgoing mail passes validation checks.

and I am all in favour of improving email security, but I would have thought that any email from @account.f9.co.uk should pass those checks anyway?

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

plusnettony — Tue, 17 Feb 2026 15:37:09 GMT

Just to clarify, we need the actual from email address so we can do some further checks. If you can confirm this, and it's a local policy 550 issue, we can resolved for you. As @Leanne_T said, we'd then need virus checks/change of password done, so I can then log in and remove the block.

From what I can see from the thread though, I can't see the actual from address which is essential. If you could PM Leanne or me this, I'll take the next steps.

Of course, if this is not a local policy issue, other advise may follow.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

balvack2 — Tue, 17 Feb 2026 15:42:03 GMT

Hi @plusnettony I sent PMs to @Leanne_T with the email addresses I'm trying to send from. I believe I can't / shouldn't state them here due to forum rules?

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

Leanne_T — Tue, 17 Feb 2026 15:47:52 GMT

Thanks @balvack2

I have received these thanks very much.

I will get back to you once I have more information

Leanne.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

balvack2 — Tue, 17 Feb 2026 15:51:22 GMT

Thanks for that @mavison

I can't see how an alias would work - granted I've not looked into it and it's decades since I set up mail systems so I'm definitely rusty on that side of things. But I'm guessing, wouldn't an alias appear to come from alias@plusnetaccount.plus.com rather than alias@mydomain.co.uk ?

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

balvack2 — Tue, 17 Feb 2026 15:55:53 GMT

Thanks @Leanne_T

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

plusnettony — Fri, 20 Feb 2026 09:10:24 GMT

Apologies if you've answered this, but does it work from your plus.com address?

If so, the issue is more than likely down to us not hosting your domain name. In this instance, you ought to be using the domain hosting companies outbound mail server.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

balvack2 — Fri, 20 Feb 2026 11:35:51 GMT

@plusnettony It does work from the mailbox that has plus.com as an alternative identity on webmail.greenby.com

However the issue is that this worked fine for years before the migration and I don't see how I can configure either plusnet or greenby webmail to use a different outbound mail server.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

offscotland — Fri, 20 Feb 2026 12:57:39 GMT

Friday 12:55 Cannot send email via Webmail. Getting the 'Sending Message' and it sits there with the scrolling wheel...

It is not my computer as I can do other useful things while watching the webmail window.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

plusnettony — Fri, 20 Feb 2026 13:08:45 GMT

I'm not sure the migration is related to this, but I'd be tempted to find the SMTP server name of the domain host as I'm sure that will resolve the issue.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

plusnettony — Fri, 20 Feb 2026 13:23:06 GMT

@offscotland wrote:

Friday 12:55 Cannot send email via Webmail. Getting the 'Sending Message' and it sits there with the scrolling wheel...

It is not my computer as I can do other useful things while watching the webmail window.

Did it come up with any error message? I've been in recently and not replicated.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

offscotland — Fri, 20 Feb 2026 13:41:35 GMT

Just tried again.

It took 15 seconds from pressing 'Send' before I got a result. Previously I gace up after 60 seconds.

Generally it seems really slow lately.

Ian

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

plusnettony — Fri, 20 Feb 2026 15:51:39 GMT

Thanks - I'll do some testing and keep an eye on it.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

pvmb — Mon, 23 Mar 2026 07:05:53 GMT

I use the Greenby Roundcube webmail interface exclusively and have also experienced a few glitches since transfer - including, I believe, this SMTP Error 550 issue. My experience, so far, is that they are short lived glitches rather than time extended problems. Trying again after a short pause or so usually seems to resolve the situation.

I'm hoping these current issues are associated with current demands on the servers from the ongoing work of email accounts transfer to Greenby and will settle down when that work is completed. Let's not forget the original Plusnet webmail email system was far from glitch free!

There is also evidence of wider email problems affecting ISP other than Plusnet, which has been associated with Microsoft.

Re: Cannot send/reply from webmail - SMTP Error (550) 5.7.1 unknown sender - plusnet & greenby i

balvack2 — Fri, 03 Apr 2026 10:22:27 GMT

@Leanne_T @plusnettony Just to confirm we've resolved this problem by migrating to a reliable email provider. Like many others we're very disappointed in the 'service' provided by Greenby.