In your sendmail.mc add:
Then rebuild sendmail.cf and restart sendmail. In Debian you do this by running >sendmailconfig. In CentOS you run /etc/mail/make followed by service sendmail restart. Other >systems have other ways to do this.
Thanks for posting this, and it definitely got me on the right track. However, I tried this, but it was still not working for me. As a side note, we were getting rejections from Google mail servers because we didn't have a reverse DNS entry for IPv6. This is definitely related to our VPS's configuration, and not our DNS configuration, because the numeric reverse DNS entry has to go to whoever owns the block of IP addresses, and that's my VPS.
Anyway, as far as I can see, there is no way to specifically turn off sendmail using IPv6 as the client. The slightly tricky resolution I came to was to specify the IPv6 options to use an IPv6 compatibility format of an IPv4 address.
e.g. if your IPv4 address is 188.8.131.52, specify:
Worked a charm for me, and as far as I can see, this hasn't been documented anywhere.