Summary / TL:DR: This article outlines a known issue with pre-headers not populating in the Windows Outlook client when sending a campaign using a Sparkpost adapter.
This has only been observed with the Windows Outlook client.
Issue
Currently, when sending a campaign using a Sparkpost adapter and the email creative contains a pre-header, the pre-header does not appear when using the Windows Outlook client.
However, the pre-header will populate in other email platforms like Gmail, Yahoo, MacOS Outlook Client, etc.
Microsoft forum that outlines the issue: https://answers.microsoft.com/en-us/outlook_com/forum/all/preview-preheader-text-missing-bug/03c560fb-a00c-44c1-8b33-42bc519d3373
Steps to Replicate Issue
Locate a email creative (template) that contains a pre-header
Send yourself a test email using a Sparkpost adapter using your work email (Outlook) AND to a non-Outlook mailbox such as Gmail or Yahoo
Confirm that you received the mailings in your inboxes
The Windows Outlook client should not contain the pre-header
The non-Outlook mailbox should contain the pre-header
Repeat steps 1-3 using a Momentum adapter instead of Sparkpost
The pre-header should populate in both the Windows Outlook client and the non-Outlook mailbox
Linked Issues
WTSM-216338 - Getting issue details... STATUS
WTSM-201962 - Getting issue details... STATUS