Subdomain Email Problem

I’ll start by saying I am using Yahoo hosting (I know a lot of you will say to switch hosts but I probably won’t). Everything is working just fine and I am able to customize my permalinks. However, when I want to setup a subdomain so that my links look like this:

https://subdomain.domain.com/year/month/day/post

I will not get any emails. I don’t get emails when someone posts a comment and if someone tries to request a lost password it will say “Your host must not support the mail() function”. The weird thing is that it all works fine if I change it so all of my address are like this:

https://www.domain.com/subdomain/year/month/day/post

Does anyone have any ideas how I can manipulate the mailing to get this to work? Thanks for your help!