Yes, you can receive mail using SMTP, either via an ISP or directly if you host your own mail domain.
Softalk Mail Server always listens out for mail on the SMTP port. This is port 25 by default, but the port can be changed in the Advanced page of the ISP properties. The Interfaces page of the Settings properties specifies which interface Softalk Mail Server listens on, either All interfaces or a specific IP address. If All interfaces is specified then Softalk Mail Server listens for an SMTP connection from the internet (if connected) as well as the local network.
If you host your own mail domain then you can configure the DNS for your domain (MX records) to direct mail to your Softalk Mail Server server. If you use a router then you may need to configure this to forward incoming traffic on port 25 to your Softalk Mail Server server computer. You should check that your firewall does not block port 25.
Alternatively, if your ISP can push mail to your Softalk Mail Server server by SMTP then you will just need to make sure that the Softalk Mail Server SMTP server is accessible from the internet. Before mail is pushed down by SMTP, the ISP will usually require that Softalk Mail Server first connects to the ISP’s SMTP server. This is done each time Softalk Mail Server connects to the ISP on its connection schedule. Note, it will be necessary to delete the POP3 server address from the Servers page of the ISP properties in order to stop Softalk Mail Server receiving mail via POP3.