SMTP Mail Sending Problems

Hi Guys, any help is appreciated.

I'm running the latest osTickets v1.9.14
I'm having some issues when trying to send emails through SMTP.

Unable to email via SMTP:outlook.office365.com:25 [sales@part-box.co.uk] Failed to set sender: sales@part-box.co.uk
[SMTP: Invalid response code received from server (code: 530,
response: 5.7.57 SMTP; Client was not authenticated to send anonymous
mail during MAIL FROM)]

When I reply to a Gmail/Office 365 email address, it all works fine.
Sending to a hotmail address just doesn't work and I get the above error in my logs.

the address sales@part-box.co.uk is a shared mailbox, but I have definitely had it sending/working to all email addresses in the past.
It's just randomly stopped working to some address'

I ran a diagnostic test email in osTickets and it came back fine. I've also tried other email addresses with office365 liscenes, still no luck.

Could this be a hotmail retrieving issue?

Default MTA: sales@part-box.co.uk
SMTP status: enabled
Hostname: outlook.office365.com
Port: 25
Authentication: No

Running PHP Ver 5.6.19

Kind Regards,
Sam