2.2.9 [RFC3501] Section 5.3, Response When No Command in Progress
The specification states: "Server implementations are permitted to send an untagged response (except for EXPUNGE) while there is no command in progress. Server implementations that send such responses MUST deal with flow control considerations. Specifically, they MUST either (1) verify that the size of the data does not exceed the underlying transport's available window size, or (2) use non-blocking writes."
Microsoft Exchange Server 2007, Microsoft Exchange Server 2010, Microsoft Exchange Server 2013, Microsoft Exchange Server 2016
Microsoft Exchange Server can send an untagged response when there is no command in progress. Microsoft Exchange has mechanisms to manage flow control. Any untagged responses that Microsoft Exchange sends are brief and, therefore, fit into most MTA windows.