emerge -av postfix
These are the packages that would be merged, in order:
Calculating dependencies... done!
[blocks B ] mail-mta/ssmtp (is blocking mail-mta/postfix-2.2.10)
[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
[ebuild N ] dev-libs/libpcre-6.6 USE="-doc" 544 kB
[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
Total size of downloads: 544 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