I keep getting bounce warnings from this list, apparently because of a problem 
with Yahoo.  The bounce messages say,

"host gmail-smtp-in.l.google.com[74.125.70.26] said:
    550-5.7.1 Unauthenticated email from yahoo.com is not accepted due to
    domain's 550-5.7.1 DMARC policy. Please contact administrator of yahoo.com
    domain if 550-5.7.1 this was a legitimate mail. Please visit 550-5.7.1
    http://support.google.com/mail/answer/2451690 to learn about DMARC 550
    5.7.1 initiative. ro6si3093371igb.36 - gsmtp (in reply to end of DATA
    command)"