topic Unable to send e-mail via WebMail: SMTP Error (550): Failed to set sender "{alias}@{user}.f9.co.uk" in Email

Unable to send e-mail via WebMail: SMTP Error (550): Failed to set sender "{alias}@{user}.f9.co.uk"

DazB — Sun, 12 Apr 2026 15:04:55 GMT

Good afternoon,

I have been receiving the following error for over a month when attempting to send e-mails, I thought this was just a transient error but it appears to be a permanent issue which has gotten worse over time.

Error: SMTP Error (550): Failed to set sender "{alias}@{user}.force9.co.uk" (5.7.1 Unknown sender).

Domains tried: f9.co.uk, force9.co.uk

Webmail Portals used: plus.net, greenby

Historically I was able to send with any {alias} using f9.co.uk or force9.co.uk via the plus.net webmail & greenby

Then about 8-10 weeks ago to get a mail through I had to switch from my usual force9.co.uk addresses to f9.co.uk

Then about 4-6 weeks ago I could only get a mail through using {user}@{user}.f9.co.uk and that is still the case today.

Does anyone know why this is happening and can help?

Re: Unable to send e-mail via WebMail: SMTP Error (550): Failed to set sender "{alias}@{user}.f

MisterW — Sun, 12 Apr 2026 15:19:54 GMT

@DazB to send from an alias address , that address must be explicitly defined as an alias of a mailbox.

Re: Unable to send e-mail via WebMail: SMTP Error (550): Failed to set sender "{alias}@{user}.f

DazB — Sun, 12 Apr 2026 15:32:55 GMT

Thank you so much! This fixed the problem.

Full instructions for anyone else who has this issue:

- Login to the Greenby Portal

- Navigate to 'Email'

- In the 'All Email' section click on the 3 dots to the right and choose 'Aliases'

- Add the alias you wish to send from (don't worry about the domain, it will enable for all)