Case sensitivity of email addresses

[#1]

This didn't happen to me but I thought I would flag it up.
As I understand the email system generally the system ignores upper and lower case differences so an email to fred@username.plus.com is treated the same as FrEd@username.plus.com
However setting Outlook Express login to username+FrEd fails whereas username+fred works.
From the help system

Quote

Enter your PlusNet username and password. Important Your username will be all lowercase letters, your password is case sensitive.

This lower case limitation appears to apply to the mailbox name after the username. Is this correct and if so why do we have this limitation and second  I haven't found the restriction on mailbox name documented.

[#2]

I think it treats them the same As web site addresses, it not peculiar to OE, Thunderbird does the same, it just the way that PN set up there User and Password.  Strange that the new Thompson routers WAP is all upper case

[#3]

Quote from: Oldjim

From the help system Quote Enter your PlusNet username and password. Important Your username will be all lowercase letters, your password is case sensitive. This lower case limitation appears to apply to the mailbox name after the username. Is this correct and if so why do we have this limitation and second  I haven't found the restriction on mailbox name documented.

It's not unusual for a username to be case sensitive. Surely you've just quoted where it's documented? After all, from a mailbox perspective 'account username+mailbox name' *is* your mailbox username (if that makes sense!).
Configuring the actual email address in your Outlook Express account settings to use upper and lowercase letters shouldn't make a difference.

[#4]

Bob,
I didn't make myself clear so I just tried to create a new mailbox Fred@username (note the capital) and it wasn't accepted with the error message as below which doesn't mention use of upper case being not allowed although it is inferred as the password can be upper case.
My question is why can't the mailbox use upper case
Username removed    - thanks jelv

[#5]

Quote from: Oldjim

which doesn't mention use of upper case being not allowed

On the other hand it does not say that upper case is allowed

Quote

letters (a-z)

Because of mixed (Windows/unix) systems that are used with e-mail its best to keep things lower case, as Windows system tend to be case insensitive and unix case sensitive.

[#6]

Plus if you look carefully it says  username (a-z)  Password (a-z)(A-Z)