HELP - I am unable to send e-mail's from my @btinternet.com to .biz account

[fishface3]

I am unable to send e-mail's from my @btinternet.com to .biz account.

I have Received emails OK from the .biz address into my @btinternet account.

I have been able to send the to .biz  address from my bt.com address no problem.

Any ideas please ?

Clive