Possible SSRF in email server settings(SMTP mode)

How it was found

Description: vul address https://demo.nextcloud.com/xxx/settings/admin/additional,when you change smtp server address ,you will get some different hints.

Reproduce steps:

1.Go to https://demo.nextcloud.com/xxx/settings/admin/additional,choose SMTP mode

2.Set server address to "172.17.1.0`,then you will get screenshot(nextcloud1.png),it means not on the same network segment

3.Set server address to "172.17.0.0`,then you will get screenshot(nextcloud2.png),it means the address not exists or doesn't open any port to access

4.Set server address to "172.17.0.1` and port to empty,then the test email will send successfully! it means this host exists and opens a smtp port

5.Set server address to "172.17.0.1and port to22`,then you will get screenshot(nextcloud3.png),it means the address exists,but can not access to the port