$ git send-email --to='"Address, Valid" <somebody@foobarbaz.com>' Comma in --to entry: "Address, Valid" <somebody@foobarbaz.com>' $ git --version git version 2.5.0
This appears to be a recent regression.
For an authority on why the current behavior is wrong, see RFC 2822 section 3.4, which defines mailbox and mailbox-list as well as section 3.2.6, which indicates that a phrase can contain a quoted-string.
--Andy