Emails use main multi-site in url instead of mapped domain

Hi, perhaps I set something wrong but I am finding that emails generated by the subscribe by email plugin contain a link to the "sub site" of my multi-site instead of the link to the mapped domain - example:

Here is a copy of what I get in the generated email from one site:

Dear Subscriber,

Vancouver Montessori School has posted a new item: VMS Newsletter

You can read the post in full here:

The URL to read the post should be

Any ideas?