Hi @Prabhjot Singh
Welcome to our forum!
Please kindly note that Microsoft Q&A forum recommends asking one question per thread so we can better focus on one question.
how can we implement bandwidth constraints for 12,000 devices when new .ost files are created and begin downloading emails?
As far as I know, there is no limit requirement regarding network bandwidth in Outlook. For the best performance, we recommend that you refer to this article: https://video2.skills-academy.com/en-us/deployoffice/fieldnotes/performance-recommendations.
For the second question, I noticed that you have already raised it in another thread, please focus on this thread: Migrating Devices Across Domains: Handling Old and New .ost Files.
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