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<****@yonsei.ac.kr>: host aspmx.l.google.com[2404:6800:4008:c03::1b] said:
550-5.7.1 [2001:320:11:248:546f:4bff:fef8:c9] Gmail has detected that this
550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR
records 550-5.7.1 and authentication. For more information, go to 550
5.7.1
https://support.google.com/mail/?p=IPv6AuthError
41be03b00d2f7-78e41c8cf4dsi11013936a12.313 - gsmtp (in reply to end of
DATA command)