Delivery and limits solve different problems
Your delivery address answers where to send a message. A limits identity answers which addresses your anti-abuse policy groups together. Store those concepts separately. Rewriting a customer's delivery address to match a deduplication rule can change login expectations and make later support work confusing.
Apply provider rules narrowly
Google says dots do not distinguish consumer Gmail addresses. Its documentation also explicitly says dots matter for work, school, or organization addresses. So a rule that removes dots from every address, or every domain hosted by Google, goes beyond that documented behavior. Preserve custom-domain dots unless you have a justified, provider-specific rule.
Plus stripping is a policy choice
Disban's signup-limit policy strips a plus suffix and groups Gmail and Googlemail spellings. The alias explorer makes that policy visible. On custom domains, plus stripping is an anti-abuse assumption, not proof of equivalent delivery. Some mail systems can use different addressing rules. Explain this tradeoff and keep a route for legitimate account-access problems.
Enforce the rule where accounts are created
A check performed only when a verification email is requested is incomplete. Two requests can receive codes before either creates an account. Recheck the identity inside the transaction that inserts the user. Keep normal authentication tied to the stored address; a shared limits identity must not become permission to log in as another account.
Know what normalization cannot do
Different catch-all addresses, separate mailboxes, and independent relay aliases can still belong to one person. Deleting an account can also change what history is available for limits. Canonicalization closes a specific class of duplicate-string problems. It does not prove personhood. Protect valuable rewards with atomic grants, explicit caps, and a documented deletion policy as well.
Source for provider behavior: Google's consumer Gmail dot-addressing guidance. Product-policy guidance is Disban's interpretation.