Do not append @steamcommunity.com to steamid64.

[LindenCX]

Identified a client running this plugin that was bouncing emails to valvesoftware.com, upon troubleshooting, this plugin appends @steamcommunity.com to the steamid64 value, which the server will detect as a valid email address, but it will bounce on valve's side.

https://github.com/floofytsuna/Steam-Login-MyBB-1.8/blob/995f64cfeda9a53ea4fbf9a6889f1c2e3f6e2c04/inc/plugins/steamlogin.php#L439

The simplest option would probably be to require users to enter an email, and have a custom field for the steamid64 value.

Here's an example of the bounced mail from Valve.