Resurrecting an old bug report (with some follow-ups) which is also listed as a known issue:

When sending data to multiple recipients, curl will abort and return failure if one of the recipients indicate failure (on the "RCPT TO" command). Ordinary mail programs would proceed and still send to the ones that can receive data. This is subject for change in the future.

@bagder - what is your position on this? E.g. would a non-breaking change to fix in v7.6x be acceptable?