[fshr]

Can you give a couple examples? I can't think of any accounts that don't allow an email change.

[justsomehnguy]

Not everything is a phpBB forum.

Also:

> Select Google Account email. If you can't open this setting, it might not be possible to change your email or username.

> If your account's email address ends in @gmail.com, you usually can't change it.

https://support.google.com/accounts/answer/19870

[saagarjha]

Say, for example, systems that directly expose your email address as the identifier that can be used to reach you. "Send money to [e-mail] [using our service]" kind of sites.