Can't set up new email address that start with a name - why?!

Can't set up new email address that start with a name - why?!

Appreciate any help with the below.

I have been setting up a series of email addresses through 'Send Mail As'.  

While I have been able to set up addresses starting with admin@ / sales@ / info@ etc whenever I try and set up an email with certain words or a name (eg. rachel@) I am not sent a verification code.

Does anyone know why this might be happening?  I have set email addresses up using the Zoho SMTP. Is this even right?

Many thanks in advance.

Rachel