Global USE flags

Flag Description Packages
mysql 2
pam 3
postgres 2
selinux 2
sqlite 3
ssl 2
syslog 2

Local USE flags

mail-mta

exim

Flag Description
arc Adds support for Authenticated Receive Chain (ARC)
berkdb
dane Adds support for DNS-based Authentication of Named Entities
dcc Adds support for Distributed Checksum Clearinghouse (DCC)
dkim Adds support for DomainKeys Identified Mail (DKIM)
dlfunc Install local_scan.h header to compile separate dlfunc libraries
dmarc Adds support for DMARC
dnsdb Adds support for a DNS search for a record whose domain name is the supplied query
doc
dovecot-sasl Adds support for Dovecot's authentication
dsn Adds support for Delivery Status Notifications (DSN)
experimental_dmarc
experimental_spf
gdbm
gnutls
gsasl Adds support for GNU SASL authentiction
idn
ipv6
ldap
lmtp Adds support for lmtp
maildir
mbx Adds support for UW's mbx format
nis
perl
pkcs11 Require pkcs11 support in <pkg>net-libs/gnutls</pkg> with USE=gnutls
prdr Adds support for Per-Recipient Data Response
proxy Add support for being behind a proxy, such as HAProxy
redis Adds support for querying <pkg>dev-db/redis</pkg>
sasl
socks5
spf Adds support for Sender Policy Framework
srs Adds support for Sender Rewriting Scheme
tcpd
tdb Use <pkg>sys-libs/tdb</pkg> for internal database storage (such as hints database)
tpda Adds support for Transport Post-Delivery Actions

sys-apps

busybox

Flag Description
debug
extra
livecd
make-symlinks Create all the appropriate symlinks in /bin and /sbin.
math Enable math support in gawk (requires libm)
mdev Create the appropriate symlink in /sbin and install mdev.conf and support files
sep-usr Support a separate /usr without needing an initramfs by booting with init=/ginit
static Make the system rescue shell (/bin/bb) static so you can recover even when glibc is broken
systemd Support systemd

sys-cluster

csync2

Flag Description
xinetd

www-apps

gitea

Flag Description
acct User and group management via acct-*/git packages
gogit (EXPERIMENTAL) Use go-git variants of Git commands.
pie