Don't know when it disappeared but I can't find the setting to make Mail NEVER automatically delete emails - whether in Trash or a Folder/Mailbox. I updated to OS 10.13.15 a few months ago and only recently noticed some emails are gone.
MacBook Pro 13", macOS 10.13
In Mail>Preferences>Accounts, highlight the Account, Mailbox Behavior tab.
In Mail>Preferences>Accounts, highlight the Account, Mailbox Behavior tab.
Your email provider may automatically purge message in the trash after a period of time. If you don't want them deleted, then don't put them in the trash.
Do NOT Delete emails
