Please i would be grateful if someone help me in " email alert " in the case of email alert in graylog 2.3,
I obtain this error !
@Nihed.Attia
Hello and welcome
Did you open that port through your firewall?
yes I open the port 587 but it still the same error

Try to enable TLS encrypted connection, use your real gmail account in from, and correct smtp server address:
transport_email_hostname = smtp.gmail.com
transport_email_use_tls = true
transport_email_from_email = your_real_gmail_account@gmail.com
https://support.google.com/mail/answer/7126229/check-gmail-through-other-email-platforms?hl=en
If not working, try to create local postfix server which will relay mails to gmail.