This is currently not possible: the SMTP appender has a buffer where it captures log events before emailing them, but it will only capture the events that are configured for the target level (ERROR in your example). If you changed this to INFO, you would get email notifications for all INFO-level log messages (not just the ones preceding an ERROR-level message).
You can raise this as a feature request on the log4j issue tracker or log4j-user mailing list.
Correction: I was wrong, wrong, WRONG!
STMP Appender does have a buffer but it is meant to capture the last X (512 by default) INFO, DEBUG, TRACE-level messages that preceded the ERROR log event. So it is supposed to work like you expected (like the log4j-1.x SMTP appender works).
The SMTP Appender will fire an email when it gets an ERROR (or more severe)-level log event. So in your config you should not only send ERROR-level log events to this appender (or you'll miss the INFO, DEBUG, TRACE events that precede it).
In your config, change <AppenderRef ref="AsyncMailer" level="error"/> to <AppenderRef ref="AsyncMailer"/>.
That should fix the issue. If you still experience problems, someone else reported a similar issue and apparently found a workaround by adding a ThresholdFilter to the configuration:
<Appenders>
<SMTP name="Mailer" suppressExceptions="false"
subject="${subject}" to="${receipients}" from="${from}"
smtpHost="${smtpHost}" smtpPort="${smtpPort}"
smtpProtocol="${smtpProtocol}" smtpUsername="${smtpUser}"
smtpPassword="${smtpPassword}" smtpDebug="false" bufferSize="20">
<ThresholdFilter level="debug" onMatch="NEUTRAL" onMismatch="DENY" />
<PatternLayout>
<pattern>%d{dd-MM-yyyy HH:mm:ss,SSS} %5p %m%n</pattern>
</PatternLayout>
</SMTP>
<Async name="AsyncMailer">
<AppenderRef ref="Mailer"/>
</Async>
</Appenders>
<Loggers>
<Root level="info">
<AppenderRef ref="AsyncMailer" />
</Root>
</Loggers>
