You have configured the KAV alerts to tell you when definitions are out of date.
You are receiving emails about this even for machines that are off-line, in some cases, off-line for maintenance or off-line long-term due to known reasons.
The definition alert only specifies that it will send an alert if the definitions are out of date, it does not specify that this is limited to on-line machines.
An agent being off-line does not change the definition dates - they are out of date whether the agent is on-line or not. The Kaseya server itself performs this check, comparing the last reported definition date with the current time - if the dates are too far apart, then the alert is generated.
WORKAROUND / RESOLUTION
This is by design, as such, there is no "fix" planned for this
Kaseya already has a couple of ways of handling alerts for machines that are off-line for known reasons and for machines where you do not want to receive alerts during certain times.
Agent Off-line for scheduled maintenance
On the Monitor Tab -> Suspend Alarms page you can specify a start time and duration of scheduled maintenance. During this time, any alerts created by the agent will be ignored - no emails will be sent, no tickets created, etc. This is ideal if you are perform maintenance on a weekly basis, and know that the maintenance may trigger an alarm.
For example, you reboot a server every Sunday night and do not want off-line alerts generated during this maintenance work.
Agent Tab - > Suspend Agent. This will stop the agent checking in and also stop any alerts. This is typically used when a machine is off-line for extended periods, or you do not want the Kaseya agent to be performing any tasks during a given period.
Further information can be found about suspend alarms and suspend agents, can be found here - https://helpdesk.kaseya.com/entries/93298607-What-is-the-difference-between-Suspend-Alarms-and-Suspend-Agent-