The access request feature allows people to request access to content that they do not currently have permission to see. As a site owner, you can configure the feature to send you mail when someone requests access to a site. You can then choose whether to approve or decline their request. If you approve the request, you can also specify the specific level of permission you’d like to assign to a user.
However, if your company has a large amount of SharePoint web’s it’s very difficult to keep track whose e-mail address is configured to receive access requests.
The script below enumerate each web and list our the email address to request access.
# This script lists Access Request mail for each site
Add-PSSnapin microsoft.sharepoint.powershell -ErrorAction SilentlyContinue
$site = Get-SPSite (Read-Host "Please enter site url")
foreach ($web in $site.allwebs)
{
$url = $web.url
$access = $web.RequestAccessEmail
write-output "Web site is: $url; Access Request Mail is: $access"
}