How do I bring a server problem to AT&T's attention?

Today I discovered that a problem that started within the last two days is caused by AT&T/Yahoo?

When using a 3rd part mail client (in my case Thunderbird) to send email via the ATT server, the message is received with a set of ?? taking the place of the 2nd (and all following) consecutive spaces.

I've verified the problem stems from the AT&T/Yahoo server by sending test messages written in Thunderbird using another account that uses another server (in my case Just Host).  Only AT&T causes the problem.