If the server insists on using encryption there's really no way around that.
As I see it, you have a few options:
1. Find a different mail server to relay your messages through that does not require TLS.
2. Use OpenSSL, GNUTls or another similar library to add encryption capabilities to your application so you can talk to your current mail server.
3. Set up your own mail server with Postfix, Sendmail or similar and then relay messages via that.
4. Use an existing commandline mail program like mailx to send your messages and invoke it from your program via system() or similar.
But anyway, this no longer has much (anything) to do with SFML...
