How do you have the account configured in outlook?
Are you using Exchange? Are you using "Cached Exchange Mode"?
What period do you have set for "Download email for the past:"?
If so was the email received before this time period?
If all the above is true then its likely due to outlook retrieving the email in online mode/view.
With regards to your question in the comments "Is there any reason not to increase the period to the maximum permitted period, which is 'All' (several years)?."
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There is several factors to take into consideration, if the mailbox is reasonably small and has a low number of emails incoming / outgoing then this shouldn't be an issue.
However if the mailbox has a considerable number of emails (100K+) or is considerably large in size (20GB+ could easily cause issues...) or has a large number of sub-folders then the hardware of the computer may not necessarily be able to handle the mailbox too well without specific components (SSD for quick R/W operations, decent CPU for indexing and general task performance). We have a number of large mailboxes that we control client side by reducing the cached amount to 3 months and advise users to use OWA if they require any further than this as the performance of Office 365's OWA out performs retrieving this data locally.
2How do you have the account configured in outlook? Are you using Exchange? Are you using "Cached Exchange Mode", what period do you have set for "Download email for the past:". If so was the email received before this time period, if this is the case then its likely due to outlook retrieving the email in online mode/view. – CraftyB – 2020-01-23T17:20:15.177
Is there any reason not to increase the period to the maximum permitted period, which is 'All' (several years)? I think the issue is a slow connection based on this.
– KAE – 2020-01-23T19:05:52.5531If you set Outlook to download email for 1 year, Outlook will only synced 1 year's emails to your ost file (local). Then if you click "search on server" when searching and click the results older than one year, Outlook will take a while to get these messages from server. You can try starting your Outlook in online mode and you will find that everything becomes slow because you are access your account directly from server. – Aidan – 2020-01-24T07:02:02.637