I know this was discussed in the linked thread, but I wanted to copy the information here for anyone here. Please see Tim's response on the linked thread:
"Webmail doesn't have the ability to modify some items but does maintain the state in the background for clients and protocols if supported by the protocol/client.
If you used EWS to migrate an account with the BETA and then used Outlook Windows or Mac that data would be available.
OWA is missing about 50% of what is in Outlook but maintains state in most cases so clients can manage pieces that aren't offered in OWA. We do something similar in SmarterMail.
EAS does not support these attributes (protocol limitation not us).
Mac Mail and other clients do not support these attributes even though they use EWS.
If you test further and see issues that contradict what I'm expecting based on the work we have done, please let me know."
As a quick follow up to Tim's comments... I did some testing, and I found a small issue with this behavior. Exchange/Outlook have 7 flag states (Today, Tomorrow, This Week, Next Week, No Date, Mark Complete and Custom). When you do a Mailbox Migration from Exchange, then you connect that SmarterMail account to Outlook MAPI, I found that some of those flags aren't retaining all of the info. (They are flagged with no date, rather than keeping their original dates/statues.)
I compared this to Outlook Mac and found that all flag types were preserved as expected. So this issue appears to be limited to Outlook Windows. I'll get this reported so we can get it resolved.
As an aside... I found no issues with connecting a SmarterMail account to Outlook MAPI, then removing the profile and re-adding it. All flag types should be preserved in that scenario.
Please give it a try, and let me know if you have differing results!