Since the great openssl unmask happened I caught this if fails to build with openssl-1.1.x (this is expected) but the dep dev-libs/openssl:0 still matches SLOT="0/1.1" looks like upstream made it really clear that they don't want to support openssl https://github.com/OpenSMTPD/OpenSMTPD/issues/738
builds fine with this patch from debian https://sources.debian.org/patches/opensmtpd/6.0.3p1-4/11_ssl_1.1.diff/ with dev-libs/openssl-1.1.0j haven't tested on lower openssl versions.