In the past, I have used Tencent Enterprise Email, Alibaba Cloud Enterprise Email and Gmail SMTP for free. However, during subsequent use, I found that there are various problems with free SMTP sending, such as requiring various authentications and restrictions. Frequency of sending emails, strict anti-spam policies, rejection of foreign emails, etc. I have talked about these issues in the three major reasons why I gave up Tencent Alibaba Cloud Enterprise Email.