[merdzd]

Кстати есть прямые попытки авторизации на сервере.

Код:

2018-04-12T00:23:48.968Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,3,10.0.0.8:25,118.201.203.86:49231,<,HELO EMAIL.MYFirm.ru,
2018-04-12T00:23:48.968Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,4,10.0.0.8:25,118.201.203.86:49231,>,250 EMAIL.MYFirm.ru Hello [118.201.203.86],
2018-04-12T00:23:49.370Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,5,10.0.0.8:25,118.201.203.86:49231,<,AUTH LOGIN,
2018-04-12T00:23:49.370Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,6,10.0.0.8:25,118.201.203.86:49231,>,334 <authentication response>,
2018-04-12T00:23:49.772Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,7,10.0.0.8:25,118.201.203.86:49231,>,334 <authentication response>,
2018-04-12T00:23:50.175Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,8,10.0.0.8:25,118.201.203.86:49231,*,,Inbound AUTH LOGIN failed because of LogonDenied
2018-04-12T00:23:55.178Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,9,10.0.0.8:25,118.201.203.86:49231,>,535 5.7.3 Authentication unsuccessful,
2018-04-12T00:23:55.610Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,10,10.0.0.8:25,118.201.203.86:49231,<,QUIT,
2018-04-12T00:23:55.610Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,11,10.0.0.8:25,118.201.203.86:49231,>,221 2.0.0 Service closing transmission channel,
2018-04-12T00:23:55.611Z,EMAIL\Default EMAIL,08D59DE9DFBB36F1,12,10.0.0.8:25,118.201.203.86:49231,-,,Local
2018-04-12T00:24:17.337Z,EMAIL\Default EMAIL,08D59DE9DFBB36F3,0,10.0.0.8:25,91.209.70.221:37259,+,,
2018-04-12T00:24:17.337Z,EMAIL\Default EMAIL,08D59DE9DFBB36F3,1,10.0.0.8:25,91.209.70.221:37259,*,SMTPSubmit SMTPAcceptAnySender SMTPAcceptAuthoritativeDomainSender AcceptRoutingHeaders,Set Session Permissions
2018-04-12T00:24:17.338Z,EMAIL\Default EMAIL,08D59DE9DFBB36F3,2,10.0.0.8:25,91.209.70.221:37259,>,"220 EMAIL.MYFirm.ru Microsoft ESMTP MAIL Service ready at Thu, 12 Apr 2018 03:24:16 +0300",

Что вы делаете с такими?
Баните IP/или всю маску провайдера.
Я пока баню если они в Китае зарегистрированы.