question to you --- does nop support plain SMTP port 25 with authentication?
also does it support SMTP port 25 without authentication at all?
Nop uses mailkit libaries. You can check the supported protocols on their website. Authenticated SMTP port 25 shouldn't be a problem but I haven't tried without auth before.
Note: nop 4.3 references an older library ver. 2.6.0
https://www.nuget.org/packages/MailKit/
http://www.mimekit.net/