Variable DEFAULT_PROVIDERSConst

Built-in provider rules.

These encode the well-documented, widely-relied-upon behaviors of major mail providers. They are deliberately conservative: domains are only collapsed where aliases truly deliver to the same mailbox (Gmail), and local parts are only lowercased for providers known to be case-insensitive.

Sources cross-checked while compiling this list:

• Wikipedia, "Email address" (sub-addressing section).
• aaronbassett's "Email sub addressing for different providers" gist.
• validator.js normalizeEmail provider conventions.
• Fastmail / Microsoft Learn / Proton provider documentation.

Consumers can extend or override any of these via NormalizeOptions.providers.