Hello,
I’m currently experiencing an issue sending emails from our Nextcloud instance (VM-NEXTCLOUD) to our Exchange server
Tests performed:
- Ports 25, 465, and 587 are open (verified with telnet and nc).
- Ping and DNS resolution work as expected.
- The Exchange server responds properly via telnet on port 25 and accepts SMTP commands such as HELO, MAIL FROM, RCPT TO without authentication.
- However, when using SMTP from Nextcloud, no emails are sent — whether using authentication or not, STARTTLS or SSL.
Observation:
The Exchange server accepts manual SMTP connections, but refuses or blocks connections initiated by Nextcloud, even with the correct credentials and encryption.