I am new to this so please ask if you require further info.
We use Office 365 Business Premium. It was set up a couple of months ago by a consultancy and has been working fine. They migrated our mail to Outlook/Exchange and migrated our various user accounts. User accounts are created on our domain controllers, their email address is entered in both the General tab and the proxyAddresses in the Attribute Editor tab. The latter is expressed as SMTP:firstname.lastname@example.org
Since that time I have created two new user accounts and their details have synchronised to the 365 tenancy. I assign a license to their account and can see their primary email account as that specified by in the proxyAddress entry.
This has not worked for the last user account I added to Office 365. Every time the user tries to use their email an error message is generated stating the address or password is incorrect. Tried resetting the password via the error message generated from Outlook on the web and it stated the email address was not recognised.
The only thing that was different about this user's set up in 365 was that we had one member of staff leave so I deleted their user account from 365 and disabled it on our domain controller which left a license free. I assigned that license to the account I am now having problems with.
This is completely new to me so any help will be very gratefully received.