According to the STARTTLS Spec Section 5:
A publicly-referenced SMTP server MUST NOT require use of the
STARTTLS extension in order to deliver mail locally. This rule
prevents the STARTTLS extension from damaging the interoperability of the Internet's SMTP infrastructure. A publicly-referenced SMTP server is an SMTP server which runs on port 25 of an Internet host listed in the MX record (or A record if an MX record is not present) for the
domain name on the right hand side of an Internet mail address.
However, this spec was written in 1999, and considering it's 2014, I'd expect most SMTP clients, servers, and relays to have some kind of implementation of STARTTLS.
How much email can I expect to lose if I require STARTTLS for incoming messages?