$MailParameters = @{
 From = 'Office 365 User <office365-user@domain.com>'
 To = 'external-user@outlook.com'
 Subject = 'Test from PowerShell'
 Body = "Hello World"
 SmtpServer = 'smtp.office365.com'
 Port = '587'
 Credential = Get-Credential -UserName office365-user@domain.com -Message 'Specify Office 365 user credential'
 UseSsl = $true
 }

Send-MailMessage @MailParameters