Estou usando um smarthost e tenho a seguinte configuração:
/etc/exim4/conf.d/main/00_local_macros:
DKIM_CANON = relaxed
DKIM_SELECTOR = 12345
DKIM_DOMAIN = ${lc:${domain:$h_from:}}
DKIM_FILE = /etc/exim4/keys/${dkim_domain}/12345.private
DKIM_PRIVATE_KEY = ${if exists{DKIM_FILE}{DKIM_FILE}{0}}
Em seguida, em /etc/exim4/conf.d/transport/30_exim4-config_remote_smtp_smarthost:
.ifdef DKIM_DOMAIN
dkim_domain = DKIM_DOMAIN
.endif
.ifdef DKIM_SELECTOR
dkim_selector = DKIM_SELECTOR
.endif
.ifdef DKIM_PRIVATE_KEY
dkim_private_key = DKIM_PRIVATE_KEY
.endif
.ifdef DKIM_CANON
dkim_canon = DKIM_CANON
.endif
.ifdef DKIM_STRICT
dkim_strict = DKIM_STRICT
.endif
.ifdef DKIM_SIGN_HEADERS
dkim_sign_headers = DKIM_SIGN_HEADERS
.endif