<log4net>
<appender name="SmtpAppender" type="log4net.Appender.SmtpAppender">
    <to value="user@destination.com"/>
    <from value="service@source.com"/>
    <subject value="some text"/>
    <smtpHost value="smtp.source.com"/>
    <bufferSize value="1"/>
    <lossy value="true"/>
    <evaluator type="log4net.Core.LevelEvaluator">
        <threshold value="Error" />
    </evaluator>
    <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%newline%date [%thread] %-5level %logger [%property{NDC}] - %message%newline%newline%newline"/>
    </layout>
</appender>
<root>
    <appender-ref ref="SmtpAppender" />
    <!--<appender-ref ref="AdoNetAppender"/>
    <appender-ref ref="RollingFileAppender" />-->
</root>

<targets>
    <target name="MailLogger" xsi:type="Mail"
            smtpServer="smt.source.com"
                    subject="NLog test"
            from="sombody@source.com"
            to="toaddress@destination.com"/>
    <target name="LogToFile" xsi:type="File"
            fileName="c:\temp\logs\NLogLog.txt"
            encoding="utf-8"
            layout="[${longdate}][${machinename}][${level}] ${message} ${exception}" />
</targets>
<rules>
    <logger name="*" minlevel="Info" writeTo="LogToFile" enabled ="true"/>
    <logger name="*" minlevel="Error" writeTo="MailLogger" enabled ="false"/>
</rules>

Error Error sending mail. Exception: System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure.

skipCertificateValidation="true"

<target name="RulesEngineLogMail" xsi:type="Mail"
            smtpServer="xxxxx.xxxxxx.com"
              subject="Rules Engine Log"
              skipCertificateValidation="true"
            layout="[${longdate}] - [${machinename}] - [${level}] - [${message}] - [${exception:format=toString}]"
            from="xxx@xxx.com"
            to="aaa@aaa.com"/>