The first one is caused by the difference between the mail transmission mechanism of the sender's mail server and the free mail transmission mechanism.

Second, the code of the sender's country and region is different from that of Chinese mainland. For example, the general e-mail code in Hongkong, Taiwan Province Province is big5 code, which may be displayed as garbled when viewed directly in the free e-mail.

The third type: the email software tool used by the sender is inconsistent with the email software tool you use.