This is strictly a sendmail issue, not scalix. Follow exactly the instructions on the feedback you're getting. Be sure to modify the /etc/mail/sendmail.mc properly:
Below is a sample:
divert(-1)dnl
include(`/usr/share/sendmail-cf/m4/cf.m4')dnl
VERSIONID(`setup for linux')dnl
OSTYPE(`linux')dnl
define(`confLOG_LEVEL', `9')dnl
define(`confDEF_USER_ID', ``8:12'')dnl
define(`confTO_CONNECT', `1m')dnl
define(`confTRY_NULL_MX_LIST', `True')dnl
define(`confDONT_PROBE_INTERFACES', `True')dnl
define(`PROCMAIL_MAILER_PATH', `/usr/bin/procmail')dnl
define(`ALIAS_FILE', `/etc/aliases')dnl
define(`STATUS_FILE', `/var/log/mail/statistics')dnl
define(`UUCP_MAILER_MAX', `2000000')dnl
define(`confUSERDB_SPEC', `/etc/mail/userdb.db')dnl
define(`confPRIVACY_FLAGS', `authwarnings,novrfy,noexpn,restrictqrun')dnl
define(`confAUTH_OPTIONS', `A')dnl
define(`confMAX_MESSAGE_SIZE',`20480000')dnl
TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
define(`confCACERT_PATH', `/usr/share/ssl/certs')dnl
define(`confCACERT', `/usr/share/ssl/certs/ca-bundle.crt')dnl
define(`confSERVER_CERT', `/usr/share/ssl/certs/smtpsvr.cert')dnl
define(`confSERVER_KEY', `/usr/share/ssl/certs/smtpsvr.key')dnl
define(`confCLIENT_CERT', `/usr/share/ssl/certs/smtpsvr.cert')dnl
define(`confCLIENT_KEY', `/usr/share/ssl/certs/smtpsvr.key')dnl
define(`confTO_IDENT', `0')dnl
FEATURE(`no_default_msa', `dnl')dnl
FEATURE(`smrsh', `/usr/sbin/smrsh')dnl
FEATURE(`mailertable', `hash -o /etc/mail/mailertable.db')dnl
FEATURE(`virtusertable', `hash -o /etc/mail/virtusertable.db')dnl
FEATURE(redirect)dnl
FEATURE(always_add_domain)dnl
FEATURE(use_cw_file)dnl
FEATURE(use_ct_file)dnl
FEATURE(local_procmail, `', `procmail -t -Y -a $h -d $u')dnl
FEATURE(`access_db', `hash -T<TMPF> -o /etc/mail/access.db')dnl
FEATURE(`blacklist_recipients')dnl
EXPOSED_USER(`root')dnl
DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl
FEATURE(`accept_unresolvable_domains')dnl
LOCAL_DOMAIN(`mail.domain.com')dnl
MASQUERADE_AS(`domain.com')dnl
FEATURE(masquerade_envelope)dnl
MASQUERADE_DOMAIN(localhost)dnl
MASQUERADE_DOMAIN(localhost.localdomain)dnl
INPUT_MAIL_FILTER(`clamav', `S=local:/var/run/clamav/clmilter.sock, F=, T=S:4m;R:4m;C:30s;E:10m')dnl
INPUT_MAIL_FILTER(`spamassassin',`S=local:/var/run/spamass.sock, F=, T=C:15m;S:4m;R:4m;E:10m')dnl
define(`confINPUT_MAIL_FILTERS', `spamassassin,clamav')dnl
MAILER(smtp)dnl
MAILER(procmail)dnl
MAILER(scalix)dnl