I'm aware of a similar defect in a different version, where the operation fails if there are too many dots in the domain portion of the address. I can't find any similar potential defect reports with respect to 10.0.1.12900-2, but I will file a report for you. I'll let you know what I find out.
In the meantime, you can create an email alias or just use the shorter domain (if that works) as a workaround.
By the way, have you tried a POST request? There was an older problem that recommended POST as a workaround.
Now, I've not tried post. I'm using Jeff's .NET lib - I'm running it in source code form so I could just swap out the HTTP method.
I opened a TAC case no this as I needed to give my customer a timeline for a fix: 632140967. The issue also happens in 10.5.1 (we tried upgrading as another solution approach).
Do you have any idea on how to create an alias if the email is external? Because that's what we're looking at.. an external Email to SMS service so we send to email@example.com. Naturally we have no say about what domain name we have to use.. the only part under our control is mobilenumber.
@edit: tested POST.. it returns 405 method not allowed.