Attaching a file that lives in SharePoint or OneDrive to an email should be simple, and most of the time it is, right up until the file you need is not one you have opened recently and does not show up in Outlook's suggested attachments. The method that actually works depends on which version of Outlook you are running, and the two versions currently behave quite differently.
Classic Outlook for Windows
In the classic Outlook desktop app, select Attach File from the ribbon, then choose Browse Web Locations, and pick either OneDrive or the SharePoint site you need. This opens a browser style picker showing your recent and shared locations, and from there you can either attach a copy of the file or insert it as a shared link, which is generally the better option since it keeps everyone working from the same live document instead of a static copy.
If the SharePoint site you need does not appear in that picker, an older but still useful workaround for Microsoft Teams connected sites specifically is the Groups section on the Home ribbon. Selecting Browse Groups shows the Microsoft 365 Groups you belong to, and their associated Team site files, though this only surfaces Team sites and will not show Communication sites, and depends on your organization not having restricted group visibility.
New Outlook for Windows
This is the part that has genuinely changed since we first covered this topic. In the newer, web based Outlook for Windows that Microsoft has been rolling out as the default, Browse Web Locations does not currently exist as an option. If you are on new Outlook and need to attach a SharePoint or OneDrive file you have not recently worked on, the most reliable path today is to open the file directly in SharePoint or OneDrive, copy its link from there, and paste that link into your email rather than trying to attach it from within Outlook itself.
The practical takeaway
If your organization is still on classic Outlook, Browse Web Locations remains the right tool. If you have moved to new Outlook, plan on copying links from SharePoint or OneDrive directly instead, since the in app browsing option simply is not there yet.
If your team is navigating the shift to new Outlook and running into gaps like this one, our Microsoft 365 coaching and enablement service can help smooth that transition. Get in touch if you want a hand with it.