How can an exchange account that is disbaled, send out a meeting request?

Mailbox disabled (disconnected) or just AD account disabled?

If just AD account is disabled I see two possibilities-

1- There shall be a delegate with SendAs or sendon behalf rights sending this invite intentionally or unintentionally.

2- An old meeting request where the organizer is the so called disabled user, is updated by respective user delegates.

or there is a also a possibility that a synced device (ActiveSync) involved here.

The AD account was disabled which in turn should disable the exchange account correct?

That is a wrong perception. Disabled AD account will deny the user to interactively logging in to the mailbox. However the mailbox is still functional and can receive emails. Delegates can work on behalf of the mailbox (and hence the email address).

Well I just checked the account in exchange and it appears that it is saying it is disabled..
How can I be sure it is disabled in exchange which I think it is?

simple - You send a test mail, it will not shoot any NDR. Check your tracking log, the message is delivered to the mailbox.

If the mailbox is DISABLED, then it will not appear in the EAC or Powershell. This mailbox will be in a Disconnected State if the mailbox is disabled. If it is only the AD Account that is disabled then mailbox is still active as Murali has already mentioned.

If you had calendar items that were stuck in the Outbox or delayed sending this could also be another reason why it was sent out.

Will.

forwarding users meeting request