These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.56 s (backtrack: 0/20).

[ebuild  N     ] acct-group/ssmtp-0-r2::gentoo  0 KiB
[ebuild  N     ] mail-mta/ssmtp-2.64_p12::gentoo  USE="mta ssl -gnutls" 95 KiB
[uninstall     ] mail-mta/nullmailer-2.2-r2::gentoo  USE="ssl -test" 
[blocks b      ] mail-mta/ssmtp[mta(+)] ("mail-mta/ssmtp[mta(+)]" is soft blocking mail-mta/nullmailer-2.2-r2)
[blocks b      ] mail-mta/nullmailer ("mail-mta/nullmailer" is soft blocking mail-mta/ssmtp-2.64_p12)

Total: 2 packages (2 new, 1 uninstall), Size of downloads: 95 KiB
Conflict: 2 blocks (all satisfied)