Where are the incomplete search results settings located? If I choose not to show this warning again, where would I go to change the setting?
You will find the settings in Menu > Tools > Accounts, then click on the IMAP tab for your account.
Otherwise, when using IMAP, only the message header is downloaded for new mail, so searching will only include the header and not the body. When you open a message, the body is downloaded.
I have done this, but the message pops up on every search I do. I use two IMAP Accounts and one Exchange Web Service Account. I can’t find any solution up to now.
The upcoming version 8 has much improved server search abilities. Maybe try the beta that is available in the Release History, and see if that works.