Does anyone know how to fix Error Code 17099 in Entourage for Mac 2008? Apart from getting new software, that is!

I am using Entourage for Mac 2008 (yes ... I know ...) and it has worked very well since I got it, but now it won't send mail, only receive it. The error message I am getting when I try to send mail is 'Too many messages 1.3.5.3 from 109.208.235.135 Error Code 17099'. As I only send about 6 mails a day, I can't think volume is the problem. Has anyone else had this and if so, how did they fix it, please? Thank you.