random “NULL character(s) present” error in exim

Sometimes we are getting **NULL character(s) present** errors on incoming mail. It looks like this: 2018-03-01 12:26:51.244 SMTP syntax error in "RCPT TO:" H=mail-wm0-f56.google.com [74.125.82.56]:59392 I=[185.78.30.71]:25 NULL character(s) present (shown as '?') 2018-03-01 13:07:10.371 SMTP syntax error in "RCPT TO:" H=mail-vk0-f59.google.com [209.85.213.59]:34762 I=[185.78.30.71]:25 NULL character(s) present (shown as '?') 2018-03-04 16:48:37.084 SMTP syntax error in "??EHLO 102jamz.com" H=[78.90.185.253]:50824 I=[185.78.30.71]:25 NULL character(s) present (shown as '?') 2018-03-04 17:00:30.333 SMTP syntax error in "??EHLO 1stglobal.com" H=[168.103.12.254]:43198 I=[5.101.158.68]:25 NULL character(s) present (shown as '?') 2018-03-05 11:11:42.758 SMTP syntax error in "D?TA" H=mail-pf0-f176.google.com [209.85.192.176]:42851 I=[185.78.30.71]:25 NULL character(s) present (shown as '?') We have had successful transfers from hosts with these errors. The error only happens sometimes. It has different commands (ehlo / rcpt to / data ), different hosts. At the same time there are no errors in dmesg, so it has no correlation with hardware error. Some TCP fails? Don't know, it looks like "we miss 1 byte in big package", is it real case? Having 4 mail servers for incoming and 4/4 gets this error in some time. exim 4.90.1 linux kernel 4.7.10 exim under docker enviroment. docker 1.8.3, build f4bf5c7