warning: hostname walkerj235.com

Hello very good here I am again with a doubt. I have deactivated all the pages to verify that they were not the cause of this attempt to send mail and I believe that it is the system that tries to communicate with the domain walkerj235.com to what is due if I have not automated such sending?
Feb 21 09:46:32 cloud postfix/smtpd[12521]: warning: hostname walkerj235.com does not resolve to address 91.200.12.152
Feb 21 09:46:32 cloud postfix/smtpd[12521]: connect from unknown[91.200.12.152]
Feb 21 09:46:35 cloud postfix/smtpd[12521]: warning: unknown[91.200.12.152]: SASL LOGIN authentication failed: xxxxxxxxxxxx
Feb 21 09:46:35 cloud postfix/smtpd[12521]: lost connection after AUTH from unknown[91.200.12.152]
Feb 21 09:46:35 cloud postfix/smtpd[12521]: disconnect from unknown[91.200.12.152]

first you might want to fix the MX record

mx walkerj235.com DNS Record not found

this should take care of

warning: hostname walkerj235.com does not resolve to address 91.200.12.152

but I have not made that call anywhere in that domain. It is clear that you want to connect and it gives an error ...... but how was it introduced in my system?