Short answer: in everyday email use, addresses are treated as case-insensitive. Sending to [email protected] or [email protected] reaches the same inbox at virtually every modern email provider, so capitalisation does not affect delivery.
What the standard actually says
The email specification (RFC 5321) splits an address into two parts at the @ sign: the local part (before @) and the domain (after @). Technically, the local part is allowed to be case-sensitive, while the domain is always case-insensitive thanks to DNS rules.
In practice, every major mailbox provider (Gmail, Outlook, Yahoo, iCloud, corporate Exchange servers…) ignores case in the local part as well. So [email protected], [email protected] and [email protected] are all delivered to the same mailbox.
| Address part | Standard behaviour | Real-world behaviour |
|---|---|---|
| Local part (before @) | Allowed to be case-sensitive | Treated as case-insensitive |
| Domain (after @) | Always case-insensitive | Always case-insensitive |
Why it matters for your contact list
Even if delivery works either way, mixing capitalisations creates two big headaches for marketers:
- Duplicate contacts — [email protected] and [email protected] may be saved as two separate rows, inflating your list and your bill.
- Broken segmentation — filters and unsubscribe lookups can fail if they perform a case-sensitive comparison.
Best practices
- Force lowercase on every signup form and import file.
- Run a deduplication pass when you migrate or merge lists.
- Validate the format with a simple regex before saving each address.
Keep your list clean from day one
Lowercase, deduplicate and validate every address you import. Need a refresher on the technical limits? Read our short guide on the maximum number of characters in an email address.