You can set the email signature that WHMCS adds to all outgoing email templates.
You can configure Google as your mail provider using SMTP.
You can configure Microsoft 365® as your mail provider using SMTP.
You can configure Microsoft® as your mail provider using Microsoft Entra® ID (formerly Azure Active Directory).
You can configure SparkPost, SendGrid, or Mailgun as your mail provider.
You can configure SMTP as your mail provider.