VM has managed to make this all very complicated to explain. I claim no particular email expertise but this is my understanding of VM's email changes ...
In the past, the primary login for 'My Virgin Media' (the account which allows you to see your bills, contract info, usage charges etc.) would allow you to administer any secondary email accounts (for things such as password changes on the secondary account).
The primary login for 'My Virgin Media' would also have a corresponding email address via a ntlworld. blueyonder, virginmedia etc. email address. Both the primary 'My Virgin Media' and the primary email address would use the same primary email address and account password to log in.
In the current arrangement, each secondary email account now also has its own 'My Virgin Media' account from where you have to make any changes to each secondary account (using the secondary email address and secondary account password to log in to each ‘My Virgin Media’ for each email account).
If all continues to work (without VM forcing you to change anything else) this is also how you would log in to VM webmail for each VM email account and you would administer each email account via its own 'My Virgin Media' login (using the relevant email address and account password).
In recent times though, VM has implemented security changes for some customers (meaning they cannot log above in as they did before).
VM has started requiring customers to provide a third-party email address for security reasons. If/when you have to do this, you then use the third-party email address as the user name and then your account password to access 'My Virgin Media' or webmail for each account (once a third-party email address has been imposed). You are still using/accessing the VM email service, it is just your login name which is replaced by using the third-party email address as the login name.
The process is explained below if you have to do this
https://www.virginmedia.com/dpb/help/change-my-virgin-media-email-address
In addition to the above, if you use an email client to send/receive email, VM has started using an 'app password' to connect email clients (such as Outlook, Thunderbird etc.). When you set up the email client, you use your full VM, blueyonder, ntlworld email address along with the app password to set up the account on the email client
To generate an app password, you have to log into 'My Virgin Media' (for the particular email account you are trying to set up on your email client) and go to
MyVM > Account Settings > Account details > Mailbox App password management
Finally, VM has begun enforcing some of its outbound server settings and disallowing some previous settings that worked in the past. The current/required settings are explained in the link below
https://www.virginmedia.com/help/broadband/manage-email-account
in the section 'Connecting your Email to a Client App'. If you find you can receive email but not send, it is the 'SMTP settings' in your email client which need updating.
I am sure others will correct, or add to, what I have suggested above if it is not entirely correct/accurate.
Aside from the VM changes above, default advice to anyone experiencing VM email problems would be that once you have a third-party email set up, start the process of migrating all of your email requirements away from VM and onto the third-party email address. It can take some time to achieve this but, once done, you will be free of the never-ending merry-go-round of VM email problems.
