I believe nail (heirloom mailx) is by design mailx compatible (i.e. a "drop in replacement").
However, it does not appear to provide virtual/mailx:
# dep -R virtual/mailx
Steps to Reproduce:
1. emerge nail
2. unmerge mailx
mailx will be re-emerged to fulfil requirements for virtual/mailx.
nail should satisfy any requirements for virtual/mailx.
$ grep PROVIDE /usr/portage/mail-client/nail/*
Sorry for wasting your time.
I guess it's a bug in udept, which I used to see if what was providing virtual/mailx.
Correction: It was user error; I omitted the -n option from udept. My apologies again.
# dep -Rn mailx
No problem. Closing ;)