Subject line and email address character limits

The subject line limit is 2000 characters and we have a 255 character limit for the From address.

Each recipient address (To, Cc, Bcc) can be up to 254 characters, the maximum length of a valid email address. Only the address counts, not the display name, so "Jane Doe" <jane@example.com> counts as 16 characters. Messages with a longer recipient address are rejected: the API returns error code 300, and SMTP sends show an SMTP API Error bounce.

For calculating the length we count the number of UTF-16 code points - unicode points in the range 0000 - FFFF are counted as 1, code points above that are counted as 2.