Issue sendmail: fatal: <USER> (UID): No recipient addresses found in message header

[Tom MüKo]

I get zombie processes every hour.

May  7 09:21:23 <server> crond: sendmail: fatal: <user1>(<uid1>): No recipient addresses found in message header
May  7 09:21:23 <server> crond: plesk sendmail[31400]: sendmail unsuccessfully finished with exitcode 75
May  7 09:21:23 <server> crond: sendmail: fatal: <user2>(<uid2>): No recipient addresses found in message header
May  7 09:21:23 <server> crond: sendmail: fatal: <user3>(<uid3>): No recipient addresses found in message header
May  7 09:21:23 <server> crond: sendmail: fatal: <user3>(<uid3>): No recipient addresses found in message header
May  7 09:21:23 <server> crond: plesk sendmail[31415]: sendmail unsuccessfully finished with exitcode 75
May  7 09:21:23 <server> crond: plesk sendmail[31407]: sendmail unsuccessfully finished with exitcode 75
May  7 09:21:23 <server> crond: sendmail: fatal: <user4>(<uid4>): No recipient addresses found in message header
May  7 09:21:23 <server> crond: plesk sendmail[31398]: sendmail unsuccessfully finished with exitcode 75
May  7 09:21:23 <server> crond: sendmail: fatal: <user4>(<uid4>): No recipient addresses found in message header
May  7 09:21:23 <server> crond: sendmail: fatal: <user1>(<uid1>): No recipient addresses found in message header

  ├─crond,4458 -n
  │   ├─crond,3356 -n
  │   │   └─(sh,3365)
  │   ├─crond,3357 -n
  │   │   ├─sendmail,3368 -FCronDaemon -i -odi -oem -oi -t -f st-romuald
  │   │   └─(sh,3361)
  │   ├─crond,3358 -n
  │   │   ├─sendmail,3381 -FCronDaemon -i -odi -oem -oi -t -f bmg-test01
  │   │   └─(sh,3369)
  │   ├─crond,3359 -n
  │   │   ├─sendmail,3376 -FCronDaemon -i -odi -oem -oi -t -f dom-museum
  │   │   └─(sh,3364)
  │   ├─crond,3360 -n
  │   │   ├─sendmail,3383 -FCronDaemon -i -odi -oem -oi -t -f efl
  │   │   └─(sh,3366)
  │   ├─crond,5402 -n
  │   │   └─(sh,5412)
  │   ├─crond,5403 -n
  │   │   ├─sendmail,5428 -FCronDaemon -i -odi -oem -oi -t -f st-romuald
  │   │   └─(sh,5409)

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
<user1>+  3361  0.0  0.0      0     0 ?        Zs   03:01   0:00 [sh] <defunct>
<user2>+  3364  0.0  0.0      0     0 ?        Zs   03:01   0:00 [sh] <defunct>
<user3>+  3365  0.0  0.0      0     0 ?        Zs   03:01   0:00 [sh] <defunct>
<user1>       3366  0.0  0.0      0     0 ?        Zs   03:01   0:00 [sh] <defunct>
<user2>+  3369  0.0  0.0      0     0 ?        Zs   03:01   0:00 [sh] <defunct>
<user3>+  5407  0.0  0.0      0     0 ?        Zs   08:01   0:00 [sh] <defunct>
<user1>+  5409  0.0  0.0      0     0 ?        Zs   08:01   0:00 [sh] <defunct>
<user2>+  5410  0.0  0.0      0     0 ?        Zs   08:01   0:00 [sh] <defunct>
<user3>       5411  0.0  0.0      0     0 ?        Zs   08:01   0:00 [sh] <defunct>
<user1>+  5412  0.0  0.0      0     0 ?        Zs   08:01   0:00 [sh] <defunct>
<user2>+  5899  0.0  0.0      0     0 ?        Zs   04:01   0:00 [sh] <defunct>
...

How do i find the error? Where are these message header generated? What to do next?

OS: CentOS Linux 7.4.1708 (Core)‬
Plesk: Plesk Onyx Version 17.8.11 Update #7