Running Exchange 2013 CU7 with Outlook 2007 utilizing standard settings:
IMAP Email from Outlook internal user or external user successful (usually) see below
From an application call by Security Information Systems product (Alarm Center) It simply tries to send an attachment via a validated user, it is successful although quirky, sometimes it holds it a while then sends. Sometimes we get this error: "The server responded: 421 4.4.2 Message submission rate for this client has exceeded the configured limit" and actually I just got that while sending an internal email to myself. So realistically IMAP also has some things to concern
SMTP Email from Outlook internal user or external user successful
From application sends to internal just fine, the other day we had it sending to external but dropping off an attachment that will go with it. Now it does not send at all, and nothing is left at the server even on the internal success. As I recall I could at least look it up and see that it left the server.
According to their support they tried my SMTP settings on their localized system (Alarm Control0 and it functions. So I am stuck.
RECAP: IMAP flaky send "usually" complains a lot from app call
SMTP was sending w/o attachment and was reflecting in server, now send "internally with attachment" but not externally, records no record of anything in server.