Ask your provider of smtp.xxx.com. Some are refusing machine created emails due to the danger of spam.
- fanyi.baidu.com
Only the first email is sent successfully after Metatrader5 restart.
I tried other email services but still encountered the same problem.
There is the key thread about it (with several posts only) so I am suggesting to read the thread:

- 2020.10.03
- www.mql5.com
You can install your own local E-Mail server:
https://gist.github.com/raelgc/6031306
https://www.hmailserver.com/
Installation and operation is a bit tricky, but it works.
- Martin Knafve
- www.hmailserver.com
There is the key thread about it (with several posts only) so I am suggesting to read the thread:
Thanks for your help. But these didn't fix my issue. My Metatrader5 can send email successfully in every time I restart it, but only the first email. If I want to send another email, I have to restart Metatrader5 again. I dont know what makes this problem.

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My Metatrader5 can send email successfully in every time I restart it, but only the first email. If I want to send another email, I have to restart Metatrader5 again. The error log is "connect to smtp.xxx.com failed" if I try to send the second and more emails. (Please view the attach file)
I tried other email services but still encountered the same problem.