Set the subject of email setting as {title} in calibre. When I send an epub file, the subject is automatically set to MOBI when I check the log. And this causes sending failure.
send: b'From:
xxx@163.com\r\nTo:
xxx@kindle.cn\r\nSubject: MOBI;Qi Ma Lan (Dou Ba...
reply: b'250 Mail OK queued as smtp9,DcCowABnnhnEb5Ni+VngFg--.3628S2