/etc/mail/sendmail.mc

把DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')
改成DAEMON_OPTIONS(`Port=smtp,Addr=0.0.0.0, Name=MTA')

把
dnl # TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')
dnl # define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')

改成
TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')
define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')