Fully Qualified Host Name?  If you are looking to add  your  email MX record in BigCommerce you need to know what this is.  If the store was using  www.fabbro.uk, for example,  I would put in fabbro.uk as the fully qualified host. The “mail handled by” value is supplied by hosting in the case of 123-reg  it is webmail.fabbro.uk. Other hosting might use mail.yourdomain.com, it should be documented.

Still a Problem

If it won’t accept the above  then you need to set up your domain properly on  BigCommerce first. Read here

For an example of setting up external email on BigCommerce click here