Below script:
Add-Type -Path 'C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll'
Add-Type -Path 'C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll'
$username = "[UserName]"
$securePassword = ConvertTo-SecureString "[Password]" -AsPlainText -Force
$spoCred = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $securePassword)
$context = New-Object Microsoft.SharePoint.Client.ClientContext("[https://[XXXXXXXXXXXXXXXXXX]/")
$context.Credentials = $spoCred
$toEmail ="[Email1]","[Email2]"
$toSubject ="[Subject]"
$toBody ="[BODY]"
[String[]]$email = $toEmail
$emailProperties = New-Object Microsoft.SharePoint.Client.Utilities.EmailProperties
write-host $emailProperties
$emailProperties.To = $email
$emailProperties.CC = [String[]]("[Email]")
$emailProperties.From = "[Email]"
$emailProperties.Subject = $toSubject
$emailProperties.Body = $toBody
[Microsoft.SharePoint.Client.Utilities.Utility]::SendEmail($context,$emailProperties)
$context.ExecuteQuery()