Online.communications@alerts.comcast.net

The comcast.net domain certainly belongs to Comcast, but the alerts subdomain may not belong to Comcast or maybe the email address is spoofed.  If the request is suspicious and you, rightly so, don't want to click on a link,  you can always forward the email to abuse at comcast dot net for them to investigate.  I would check out whatever account that supposedly has suspicious activity to see if there is anything to it.  You could also call the Security Assurance Team at 1-888-565-4329.  There are numerous older threads relating to this same question.