mjhjmtu

So I was playing a little with permissions in my system and then I noticed there is no permission specified for sending the file somewhere else.

I tried, as a simple user, the following command:
mail -a //etc/shadow myAddress@domain.com
I was satisfied to get a Permission Denied message, but it's still not clear what the permissions are required in order to send a file.

I mean, I use the mail command for mail protocol, but what about other commands or other protocols?

btw, the permissions for the shadow file were:

-rw-r----- 1 root shadow 1759 Oct 23 2017 shadow

There isn't one, because "sending" a file isn't really a filesystem-level operation. What the mail command does, is that it opens the file for reading, reads the data, and sends (writes) it over the network socket (probably encoded in the case of email, not that it matters). Similarly, an FTP client, scp, or any other would do the same, they'd read the file as usual.

You don't have read access to /etc/shadow, so mail running with your user id cannot open it for reading.

Linux does have the sendfile() system call, which directly copies data between two file descriptors, but that's basically the same as calling read() on the one and write() on the other fd, except that it happens within the kernel so there's less system call overhead. It, too, requires the source to be opened for reading.