Multiple maillog error messages - postfix

[MislavO]

Plesk 12 is used, latest #MU on CentOS 6.5 final.

Maillog is full of messages like:

Mar 31 14:56:52 hosting plesk sendmail[1177]: _mh_fork(): Error occured during waiting the child process with pid: 1178: No child processes
Mar 31 14:56:52 hosting plesk sendmail[1177]: Error during 'check-quota' handler
Mar 31 14:56:52 hosting plesk sendmail[1177]: Unable to get sender domain by sender mailname
Mar 31 14:56:52 hosting plesk sendmail[1176]: waitpid failed (10): No child processes
Mar 31 14:56:52 hosting plesk sendmail[1180]: Unable to get sender domain by sender mailname
Mar 31 14:56:52 hosting plesk sendmail[1180]: handlers_stderr: SKIP
Mar 31 14:56:52 hosting plesk sendmail[1180]: _mh_fork(): Error occured during waiting the child process with pid: 1181: No child processes
Mar 31 14:56:52 hosting plesk sendmail[1180]: Error during 'check-quota' handler
Mar 31 14:56:52 hosting plesk sendmail[1180]: Unable to get sender domain by sender mailname
Mar 31 14:56:52 hosting plesk sendmail[1179]: waitpid failed (10): No child processes
Mar 31 14:56:52 hosting plesk sendmail[1183]: Unable to get sender domain by sender mailname
Mar 31 14:56:52 hosting plesk sendmail[1183]: handlers_stderr: SKIP
Mar 31 14:56:52 hosting plesk sendmail[1183]: _mh_fork(): Error occured during waiting the child process with pid: 1184: No child processes
Mar 31 14:56:52 hosting plesk sendmail[1183]: Error during 'check-quota' handler
Mar 31 14:56:52 hosting plesk sendmail[1183]: Unable to get sender domain by sender mailname

I've tried quite few things suggested in threads like:

https://talk.plesk.com/threads/pppm-2612-outgoing-mail-limits-not-limiting-mail.332066/
https://talk.plesk.com/threads/ppp-...l-you-are-not-allowed-to-use-sendmail.332016/
https://talk.plesk.com/threads/pppm-2612-outgoing-mail-control-on-domains-simply-doesnt-work.332436/

including non_smptd_milter commenting in postfix main.cf and turning of completely outgoing message limit in plesk, however this continues and keeps spamming maillog (about 80% of log is only this).

Any suggestions what to do?

 

[MislavO]

New errors today after enabling yesterday DomainKeys signatures and enabling it on few domains (note that verify incoming email is disabled and key strength of keys is 1024):

Apr 1 09:41:27 hosting dk_sign[8110]: DK_STAT_SYNTAX: Message is not valid syntax. Signature could not be created/checked
Apr 1 09:41:27 hosting dk_sign[8110]: Auth_ID: [portofkotor.co.me] Signed: [No] Header List: [No]
Apr 1 09:41:27 hosting /usr/lib64/plesk-9.0/psa-pc-remote[24120]: Error during 'dd51-domainkeys' handler
Apr 1 09:41:27 hosting dk_sign[8111]: DK_STAT_SYNTAX: Message is not valid syntax. Signature could not be created/checked
Apr 1 09:41:27 hosting dk_sign[8111]: Auth_ID: [portofkotor.co.me] Signed: [No] Header List: [No]
Apr 1 09:41:27 hosting dk_sign[8109]: DK_STAT_SYNTAX: Message is not valid syntax. Signature could not be created/checked
Apr 1 09:41:27 hosting dk_sign[8109]: Auth_ID: [portofkotor.co.me] Signed: [No] Header List: [No]
Apr 1 09:41:27 hosting /usr/lib64/plesk-9.0/psa-pc-remote[24120]: Error during 'dd51-domainkeys' handler
Apr 1 09:41:27 hosting dk_sign[8110]: DK_STAT_SYNTAX: Message is not valid syntax. Signature could not be created/checked