Since changing from tesco to

Question

since changing from tesco to sky broadband i have been unable to e-mail my relatives in australia.the e-mail is not going through.getting notification of this on my computer do i now have to add au or aus after their e-mail address. have now tried to send same message over a period of over a week with the same result. the only thing that has changed from tesco is my e-mail address. can you help or suggest anything.