A 2FA Mule is a mobile phone configured to forward SMS 2FA codes via email.

This divorces 2FA from the mobile phone you carry with you and makes it possible to perform 2FA without your phone, after having your phone lost or stolen, while on an airplane, or while roaming in a foreign place with an alternate SIM card.

In my case, the 2FA mule sits in my office lab connected to mains power.

It is an unlocked Google Pixel phone with no google account and no apps installed except for "SMS Forwarder".

It is configured to forward all SMS to an email address via encrypted SMTP.