These are distributed Brute-Force Attempts from a botnet (YLMF-PC) to send via SMTP Authentication. As your version of SmarterMail (8.x) does not have the ability to block SMTP connections by their EHLO then the only options you have are as follows:
A.) Simply ignore these connections. If you are enforcing strong passwords for your users it is highly unlikely they will ever successfully connect. Everything is working as it should.
B.) Configure your Abuse Detection for SMTP to be more strict. You can do this under SECURITY > ADVANCED SETTINGS > ABUSE DETECTION and modify your Password Brute Force By Protocol rule as follows:
Detection Type: Password Brute Force by Protocol
Time Frame: 1440 Minutes
Failures Before Block: 3
Time to Block: 43200 Minutes
Description: SMTP Brute Force
Eventually IP Addresses in the YLMF-PC botnet will begin to get blocked.
Note that customers may begin to get their Outgoing SMTP blocked by the same Abuse Detection rule as if they put in a wrong password in their email client (like when they get a new phone) their email client will probably trigger the same Abuse Detection rule within 15-45 minutes.