E-mails sent using MAPI stuck in Outbox

Here’s some background information. The customer I was migrating was using Microsoft Office 2010 / 2013 on Microsoft Windows Server 2016 using Citrix XenApp 7.15, applications were deployed using automation. There was a published application that used MAPI to send e-mail. During User Acceptance Tests (UAT’s), users complained of e-mail being stuck in their Outbox when pressing send in the published application.

As we had to automate the creation of the mailbox prior to published application launch, we had to use a PRF (Outlook Profile file). Unfortunately, in my case there was one which was already created by the customer and validated.

Since you cannot reverse engineer a PRF using Microsoft’s Office Customization Tool (OCT) I was forced to manually alter the PRF. Google was not my friend as I could not find any article about the settings that were present in the PRF.

After a few tests – by removing one setting at a time from the PRF – and removing my profile, I concluded that CachedExchangeConfigFlags=0x00000100 is the entry that you want to delete from the PRF to have the mailbox in Online mode.
After setting the mailbox to Online mode the published application was able to send e-mail without getting stuck in the Outbox.