doublecaps
Can I assume you use smtp and ssmtp independently ?
Yes
If I change one of the remote PCs to use ssmtp, would it work without changing any other config?
Yes, it should do as long as ssmtp is enabled in server manager as previously advised.
ie the email client settings are (depending on your email client)
Outgoing Mail Server
tick the box for My Server Requires Authentication
Note - Do not tick the box for Logon using secure password authentication
Also you will need to change the ports
ie for IMAP
Outgoing Mail (SMTP) port 465
Also tick the box for This server requires a secure connection (SSL)
Incoming mail (IMAP) port 993
Also tick the box for This server requires a secure connection (SSL)
Note that you will only by able to relay messages via your sme server if you do use ssmtp (ie send messages to other domains via your smtp server).