How can I enter oversea mailing address? We are living oversea. I couldn't enter the mailing address for my husband because it only allows for Canadian address.

Are you filing a non-resident return? If so, you'll need to print and mail your return.

So, enter any valid Canadian address just to get past that step. Then, when you print your return to mail it, cross out the address on your T1 form and write in your actual address.

We left Canada on Dec 31, 2017. We were Canadian resident before we left. Can I file electronically in this case? If so, how should I enter the mailing address?

You should enter you actual mailing address. If it's out of the country, you'll need to print and mail.