I am experiencing the well document problem of outgoing email being denied with error VM305 and am working through the very helpful procedure suggested by @ravenstar68 to try to track down the offending device. I am the landlord of an HMO property with 5 tenants sharing the same router. Some have multiple devices, phones, Roku sticks, tablets and computers etc.
I can see why fraudulent access to servers on port 25 from listed IP addresses would be rejected, but not why valid connections on the correct port with authentication would be?
It seems it's only me that has a problem as I use Outlook and have a blueyonder email address. The tenants all seem to access their email accounts in other ways. Persuading them to co-operate in my investigation into their devices is an up hill struggle!
A temporary workaround for me has been to connect my computer to the hotspot on my phone while I am sending email from Outlook.
