Host unknown (Name server:XXXXXX no data known

[tethys ocean]

Hi,

I am use sendmail Version 8.12.11 recently I have lived such error
What is the meaning of this error mesages

The following addresses had permanent fatal errors
<tttt at ttttt.com>  
 ----- Transcript of session follows 550 5.1.2
<tttt at ttttt.com>... Host unknown (Name server:
tttt at ttttt.com: no data known





j24Ar6Vo069160: to=tttt at ttttt.com,
ctladdr=<tethys at tethys.com> (514/10), delay=00:00:00,
xdelay=00:00:00, mailer=esmtp, pri=48595,
relay=02online.de, dsn=5.1.2, stat=Host unknown (Name
server: 02online.de: no data known)



j24DeTs1089745: to=tttt at ttttt.com,
ctladdr=<tethys at tethys.com> (514/10), delay=00:00:01,
xdelay=00:00:01, mailer=esmtp, pri=32975,
relay=mail.02online.de. [163.14.26.89], dsn=2.0.0,
stat=Sent (OK id=1D7D6E-0008QO-00)




j13Ag3tT030048:to=aa at aaa.com,
ctladdr=<vvv at ccc.com> (514/10), delay=00:00:00,
xdelay=00:00:00, mailer=esmtp,
pri=31805,relay=aa at aaa.com, dsn=5.1.2, stat=Host
unknown (Name server: aa at aaa.com: host not found)


j13Ag1Uh030018: to=<aa at aaa.com>,
delay=00:00:01, xdelay=00:00:01,mailer=amavis,
pri=31435, dsn=2.0.0, stat=Sent Feb  3 12:42:03 hypatia
sendmail[30050]: j13Ag3tT030048: j13Ag3tS030050: DSN:
Host unknown (Name server: aaa.com: host not found)


j13DtgiW057676: ruleset=check_mail,
arg1=<aaa at aaa.com>, relay=wwxw.com
reject=553 5.1.8 <aa at aaa.com>... Domain of
sender address aa at aaa.com  does not exist