We are having a few issues with a few clients that have more than one account configured on their Outlook 2010, we use Office 365 here.
Some computers might have two or three accounts configured on their computers, but it won't stop asking for password for one account (different accounts on each machine), if we remove proxy configuration, works without a single issue. All email, Exchange, accounts are on the same domain.
Basically:
Accounts configured: mail1@contoso.com, mail2@contoso.com, mail3@contoso.com
Only keep asking for password of account mail3@contoso.com
Can't see this as a proxy issue since one or two accounts are working fine, but if we add another one, it does receive/send emails, but keep asking for the password as soon as Outlook is opened.
Password were removed from the vault already, issue continues.