slow mail delivery with large 365 mailboxes

we seem to have an issue with some mailboxes that have been migrated to 365
mail delivery can be very slow
these mailboxes can be around 40gig
sometimes mail has been delivered to the cell phone long before the desktop
if we removed cached mode it seems to be better
is this due to the large cache ost file
would archive fix this maybe?
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Vasil Michev (MVP)Commented:
Yes, Outlook can definitely slow down when the OST file grows too large. And yes, archiving is a suitable solution. Other than that, you can adjust the "sync slider" to download only a subset of the mailbox content, as long as you are using Outlook 2013+:

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dougdogAuthor Commented:
when archiving is turned on for the user
is this created in a different ost file or is it online only?
Vasil Michev (MVP)Commented:
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dougdogAuthor Commented:
if i adjust the slider to only download some content this badly affects the search feature in outlook?
Vasil Michev (MVP)Commented:
No, Outlook uses server-based search now for Exchange Online accounts.
dougdogAuthor Commented:
we had users running outlook in cached mode and they complained that the search did not work very well
we changed the "days to keep mail offline to all" and this seemed to help?
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