An old email can be visible while a spreadsheet or ZIP attachment still needs downloading. Before working somewhere without a reliable connection, save the specific files you need and test those saved copies rather than relying on the message list.

In Mail on Mac, open Settings, Accounts and the account’s Account Information pane. Apple distinguishes media attachments from other attachment types. For the latter, the documented options include All, Recent for the past 15 months, and None. Review the setting that applies to the account instead of assuming every attachment was cached.

Save the working files deliberately

While connected, locate each required message and confirm the sender, date and attachment version. Use File, Save Attachments, or the attachment control in the message header, to save the intended files into a named project folder.

Where several messages contain a file with the same name, keep the versions distinguishable. Include the message date or another meaningful label in the folder structure. Do not overwrite one attachment with another before comparing their contents.

Open the saved files in the applications you will use. Check that a workbook contains the expected sheets or that a document includes the required pages. If the file itself relies on online content, test that dependency separately.

Verify offline access and preserve the correspondence

After saving your other work, perform a brief offline test. Open the files from the project folder, not from a recent-items entry whose location is unclear. Restore connectivity when the test is finished.

Avoid Message, Remove Attachments as a way to clear a download cache. Apple states that for IMAP accounts this removes attachments from the mail server and they cannot be retrieved through that action. Saving a separate working copy and stripping an attachment from a message are different operations.

Keep the original messages for context and back up the project folder if the attachments form part of the work record. Record which file is the working version before travelling so an offline session does not begin with uncertainty about several identically named copies.

References: Apple: attachment download settings, Apple: saving and removing attachments.