topic Re: Can't send emails in Email

Can't send emails

Ray5 — Tue, 17 Mar 2026 19:41:59 GMT

This messgage started popping up yesterday when sending email in Outlook

The email address in account properties is the same as the shown below - [Removed]@[Removed]force9.co.uk

Task '[Removed]@[Removed].force9.co.uk - Sending' reported error (0x800CCC78) : 'Cannot send the message. Verify the e-mail address in your account properties. The server responded: 550 5.7.1 Unknown sender'

Moderator's note by Mike (Mav): Personal information removed from a public forum.

Re: Can't send emails

PhilipHeyes — Tue, 17 Mar 2026 17:52:26 GMT

I am reading reports Greenby may expect the actual email address ( not alias ) to be used as the SMTP Username.

Re: Can't send emails

mavison — Tue, 17 Mar 2026 21:48:30 GMT

@Ray5 wrote:

The server responded: 550 5.7.1 Unknown sender'

Does this post help?

Re: Can't send emails

Ray5 — Wed, 18 Mar 2026 01:53:16 GMT

It doesn't log in if I do that. The login works with my usual user name.

Re: Can't send emails

Ray5 — Wed, 18 Mar 2026 01:54:26 GMT

I don't think so, as I am not using any 3rd party stuff here - just my regular details.

I can recieve emails OK.

Re: Can't send emails

MisterW — Wed, 18 Mar 2026 14:39:38 GMT

It's not the username used to access the SMTP server that's the problem.

Its the identity that's configured in Outlook as the FROM: address.

Since the migration to Greenby , any identity used for sending email MUST be either a valid mailbox name or an alias name.

Re: Can't send emails

Ray5 — Wed, 18 Mar 2026 14:42:15 GMT

I logged into the Greenby webmail and saw that it was indeed wanting the default email account name. I'd been using an alias for so long that I thought that was the default.

In fact, that alias wasn't even in the mailbox list anymore.

I changed the sender address to the default in outlook and it seems to work now.

Re: Can't send emails

MisterW — Wed, 18 Mar 2026 14:58:10 GMT

In fact, that alias wasn't even in the mailbox list anymore.

If you actually want to send using the alias name, then you can just add that alias to the mailbox using the Greenby portal. Once added, you SHOULD be able to use that alias name to send email. Although you will probably need to use the aclual mailbox name & password to authenticate with the outgoing (smtp) server.

Re: Can't send emails

IMM — Wed, 18 Mar 2026 16:10:00 GMT

@MisterW wrote

It's not the username used to access the SMTP server that's the problem.

Its the identity that's configured in Outlook as the FROM: address.

Since the migration to Greenby , any identity used for sending email MUST be either a valid mailbox name or an alias name.

I am wary of contradicting you MisterW, however, I send emails from me@accountname.plus.com successfully - it is not a mailbox or alias. All my replies come to accountname@accountname.plus.com which is my catch-all mailbox.

The settings I use in Outlook are as below..

Re: Can't send emails

PhilipHeyes — Wed, 18 Mar 2026 16:16:04 GMT

I can't see your image / screen shot. File attachment tend to be more available than inline images.

Send a test message from : me@accountname.plus.com to your self at me@accountname.plus.com

Look very closely at what has arrived, why does the response not go back to the originator when you select Reply.

The Greenby.com portal allows the catch all to be disabled, in fact it recommends the catch all is disabled.

Re: Can't send emails

MisterW — Wed, 18 Mar 2026 16:20:56 GMT

@IMM

Yes, it will still work on a plus.com account because the smtp server (relay.plus.net) is still routed directly to the plusnet server. @Ray5 has a force9 account which is routed to the greenby smtp server.

The greenby server is more restrictive and will only allow a sending address for which a mailbox or alias exists.

In the near future, relay.plus.net WILL be routed via the proxy and so once a plus.com account is migrated , sending mail will be routed to the greenby server and the restrictions will apply.

NB @PhilipHeyes you should be able to see the image now

Re: Can't send emails

Dan_the_Van — Wed, 18 Mar 2026 16:28:06 GMT

@IMM

Have you checked "More Settings..." this is where you define the ports and encryption.

EDIT: Outgoing Server is where you define the outgoing password

You also need to remove the tick on require SPA.

Re: Can't send emails

PhilipHeyes — Wed, 18 Mar 2026 16:31:08 GMT

@IMM

In message 9 above now see the image of your Outlook configuration.

I notice that the Username: is the account name, in this box I have the email address for all of our 7x email accounts.

Re: Can't send emails

IMM — Wed, 18 Mar 2026 17:04:49 GMT

@MisterW I have been migrated to Greenby, which was (fairly) seamless. My emails continue to work as they always did. Are you suggesting that there will be a further change to the Plusnet / Greenby system which is likely to break what is working at the moment? (and that the solution will be to add an alias of me@accountname.plus.com to the default mailbox?)

IMM

Re: Can't send emails

MisterW — Wed, 18 Mar 2026 17:10:55 GMT

@IMM

Are you suggesting that there will be a further change to the Plusnet / Greenby system which is likely to break what is working at the moment? (and that the solution will be to add an alias of me@accountname.plus.com to the default mailbox?)

In a word, yes!.

Re: Can't send emails

mavison — Wed, 18 Mar 2026 18:27:45 GMT

@IMM

Even if it works immediately after migration, beware that at some time it will change to insist on aliases or mailboxes to send emails.

As @MisterW says, Force9 is already using the Greenby IMAP server which seems to impose the requirement.
However, for about a month after my (Force9) migration it was not a requirement, then it was imposed without warning around the end of January, causing me major problems.