How to Email Books and Documents to Kindle?

Ref: link

Notice that the file size must be under 50MB

• find my Kindle personal email address within Kindle iOS apps or Kindle devices
• add my personal email address to "Send to Kindle" Approved Email list
• configure sender's email login credentials (my personal email address) in Calibre to send documents from Calibre to Kindle
  • email server part in Calibre:
    • hostname: smtp.gmail.com
    • port: 465
    • username: my sender's email login
    • password: my sender's email App Passwords setup for sending from Calibre
      • this step is done as required by guide from Calibre at link
      • read more about App Passwords at this link from Google

Managing subgroups of books by nested tags

ref: link

• In metadata of book, setup tag hierarchy, separated by . (period)
  • eg. with History.Military tag, the subgroup Military is listed under the group History
• Go to Preferences → Look & feel → Tag browser and enter the lookup name tags into the Categories with hierarchical items box. Press Apply, and you are done with setting up.

Change the default viewer for Calibre to SumatraPDF for epub files

For Windows OS

• Set the default program for opening epub files to sumatrapdf
• Go to Calibre Preference
• Under Interface, select Behavior
• On the right, is a list of book formats. These are the filetype that the Calibre Default viewer will open when you click on a title in the Library browser. Disable whichever format that you don't want Calibre to automatically launch its internal viewer.
• Click Apply and exit.

The next time you click on a book, with filetype epub or pdf, the Windows default reader app, configured by you, will launch instead of the Calibre book viewer.