Thunderbird (Email and Calendar)★★★★★Stable, capable desktop email application, works well with multiple accounts including Gmail, Nextcloud, easy to set up and use but with advanced settings when you need them. FLOSS.
How Thunderbird’s Scam Detection Works (2005) (Obsolete)
Tracking down what causes Mozilla Thunderbird to label a message as suspicious, as well as how to train it to ignore emails that you know are legit.
Move Email Archives to a New AccountThunderbird, Vivaldi, Apple Mail and Outlook can move messages from one IMAP account to another. Just drag and drop! But Gmail makes it a bit more complicated.