Load-Balancing SMTP for Hosted Exchange

Hi, we are installing Exchange 2010 SP3 for Hosted Environment and decided to use F5 instead of NLB for the CAS and HUB services. We're done with most of the services but ran into problem with load-balancing the SMTP receive connectors with SSL-Offloading. Problem is when user is configuring his account as IMAP\POP3 in outlook and configures the outgoing server authentication to use the same setting as incoming server, the outgoing server is rejecting the credentials of "john@domain.com", but if the credentials are changed into "john", everything works.

I've been informed in some forum that it doesn't work because it needs to be the LDAP domain and not the smtp email address and it is related to the encoding or something.

Once I am changing the configuration to work with NLB, I don't have this kind of issue, only when it passes the F5. Does anyone here faced it before? I cannot keep it this way, it will work only if there will be only one "john" but if there are multiple users with the same name (john@domain.com, john@mydomain.com, john@domain.net) I'm afraid that we will face a real issue.