Recently re-set password and it worked for a few days, but the last few days it's stopped

Hi NeRoZ3 

Thanks for posting and welcome to the community. Apologies for any email issues. 

Please try this for a third party client email;

Sign into the My Virgin media account for the email concerned with your VMO2ID
Select Account Settings
Select Identity & Security Settings
Select Manage your products
Select Manage email app password
Click on the blue button 'Get New Password'
Make a note of the generated password. This is what you use in the mail apps along with the Virgin Media email address as the username.