I am a noob for using client email app. I often get emails with attached images or templates like from ticketing services.
How do I get all emails synced using IMAP, fully downloaded including content, images, and attachments so that they can be accessed offline?
You can change Fetch messages up to to any size (no limit) in settings. See Account settings - K-9 Mail
Thanks.
One more, if I change phone, can I be transferred data to the new phone without need set-up and download the email again?
For example WhatsApp, when I want to change my phone, I just move all chat and media data from an external folder (Android/media/com.whatsapp) to my new phone. Can K9mail also do data transfer like that too?
No, transferring data from one device to another is currently not supported.
You can export your settings on one device and import them on another, though.