The official limits for an email address are set by RFC 5321: a maximum of 64 characters for the local part (before the @), 255 characters for the domain part (after the @) and 320 characters in total. In practice almost every mailbox provider enforces a stricter cap, usually well under 100 characters.
The official limits
| Part | Maximum |
|---|---|
| Local part (before @) | 64 characters |
| Domain (after @) | 255 characters |
| Full address | 320 characters |
| Per domain label | 63 characters |
What providers actually accept
Gmail, Outlook, Yahoo and most ISPs apply their own internal limits, often between 30 and 64 characters total. Long addresses are rejected at sign-up, and corporate filters sometimes truncate or refuse them silently — which is why [email protected] often bounces even though it is technically valid.
Are addresses case-sensitive?
The local part is technically case-sensitive but in practice every modern provider treats it as case-insensitive. The domain is always case-insensitive. See are emails case sensitive? for the details and the implications for your contact list.
Clean addresses, fewer bounces
Use Mailpro’s import validation to catch overly long, malformed or duplicate addresses. Pair it with a confirmation email so subscribers can fix typos before being added to your list.