Send email via your Gmail/Google Apps/Office 365 account

... from ISPs that don't allow port 25 traffic (or don't allow it reliably)
... or from devices that don't support authenticated SMTP

Install hMailServer (for Windows 2000 through 2012)
Settings > Protocols > SMTP > Delivery of e-mail tab...
   Number of retries: 10
   Minutes between every retry: 30
   Local host name: [your_server_name].example.com (use your own domain)

For Gmail/Google Apps use...
  Local host name: location.example.com
  Remote host name: smtp.gmail.com (or see other options)
  Remote TCP/IP port: 465
  Server requires authentication: Check
  User name: [username]@gmail.com (or Google Apps account @ your domain)
  Password: [yours]
  Connection security: SSL/TLS

For Office 365 use...
  Local host name: location.example.com
  Remote host name: smtp.office365.com (or see other options)
  Remote TCP/IP port: 587
  Server requires authentication: Check
  User name: [username]@example.com (use @ your domain)
  Password: [yours]
  Connection security: STARTTLS (Required)
    [SSL cert *may* be necessary for STARTTLS to function with Office 365]

Save

Settings > Advanced > Auto-ban
  Uncheck: Enabled

Save

Settings > Advanced > IP Ranges > Add...
  Name: insideLAN
  Priority: 20 (highest priority # here means used first, note this is opposite of MX priority)
  Lower IP/Upper IP: [the range of LAN IPs that will need to send mail, or just this server IP]
  Other...
    Anti-Spam: Not Checked
    Anti-Virus: Not Checked
  Require SMTP Authentication...
    Local to local e-mail addresses: Not Checked
    Local to external e-mail addresses: Not Checked
    External to local e-mail addresses: Not Checked
    External to external e-mail addresses: Not Checked

Save

Settings > Advanced > SSL/TLS
  Verify remote server SSL/TLS certificates: Not Checked
    (unless you like the problem where smtp.gmail.com will randomly stop accepting your messages, then leave this checked)
  Versions: Check all 4 boxes

Save

Settings > Advanced > TCP/IP ports
  Remove all except the one you need (0.0.0.0 / 25 / SMTP)

Save

  - or -

Note: I no longer recommend the IIS/SMTP service method because it has "issues" (randomly stops sending) but here are the steps if you must...
Install SMTP service (for XP, 2000, 2003, or 2008)
In IIS 6 manager
  Right-click the SMTP Virtual Server, Properties
Access tab
  Relay button
    Only the list below
      specify the IPs that may send email, including at least the IP of this server
Delivery tab
  Outbound security button
    Select Basic Authentication
      specify [username]@gmail.com (or Google Apps account @ your domain)
      and password
      TLS encryption: Check
  Outbound connections button
    TCP port: 587
  Advanced button
    Smart host: smtp.gmail.com

  - with either method you need to -

Allow port 25 inbound to this server
Add Windows firewall exception and/or disable McAfee rule which prevents port 25 use
Configure your device/application to send email to the IP of this SMTP server

[END]