I-phone e-mails

I can receive e-mails but not send, they go into my outbox, when I try to send I get flag saying “ cannot send mail the username or password for ————-is incorrect.

I have checked out settings and changed password settings but still the same happens.

I have recently upgraded bt router.

my I-phone and l-pad do not communicate now on outbox’s ie. sent e-mails on l-phone only go into phone outbox and sent e-mails on the l-pad only go into the l-pad outbox