RFC 5321 (the current SMTP spec, obsoletes RFC2821) states:
4.5.3.1.1. Local-part
The maximum total length of a user
name or other local-part is 64
octets.
4.5.3.1.2. Domain
The maximum total length of a
domain name or number is 255 octets.
This pertains to just localpart@domain, for a total of 320 ASCII (7-bit) characters.
If you plan to normalize your data, perhaps by splitting the localpart and domain into separate fields, additional things to keep in mind:
- A technique known as VERP may result in full-length localparts for automatically generated mail (may not be relevant to your use case)
- domains are case insensitive; recommend lowercasing the domain portion
- localparts are case sensitive; user@domain.com and USER@domain.com are technically different addresses per the specs, although the policy at the domain.com may be to treat the two addresses as equivalent. It's best to restrict localpart case folding to domains that are known to do this.
