emerge -tav postfix
These are the packages that would be merged, in reverse order:
Calculating dependencies... done!
[blocks B ] mail-mta/ssmtp (is blocking mail-mta/postfix-2.2.10)
[ebuild N ] mail-mta/postfix-2.2.10 USE="ipv6 pam sasl ssl -cdb -hardened -ldap -mailwrapper -mbox -mysql -nis -postgres (-selinux) -vda" 0 kB
[ebuild N ] dev-libs/cyrus-sasl-2.1.22 USE="berkdb crypt gdbm pam ssl -authdaemond -java -kerberos -ldap -mysql -ntlm_unsupported_patch -postgres -sample -srp -urandom" 0 kB
Total size of downloads: 0 kB
!!! Error: The above package list contains packages which cannot be installed
!!! at the same time on the same system.
For more information about Blocked Packages, please refer to the following
section of the Gentoo Linux x86 Handbook (architecture is irrelevant):
http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?full=1#blocked