Problem with email addresses with full stop and numbers

Hi I have and friend with a yahoo account and in the address is similar to this aa.12345@yahoo.com and it is rejected by eM Client with a message ‘The message dosn’t have any valid recipient. Please enter valid recipient in the To,Cc or  Bcc field(s).’ and an Ok button then it just rejects again.  I tried the same through Thunderbird ( Note a free and less buggy) and it works no problem, as well as if I go webmail no problem.  I think there is a long way for it to get out of betta the version 7.0.26687.0