You've got several problems in your code...
When you get "every person's email 1" you get a list of references, not a list of email addresses. To get the actual email address from a reference you get its "value".
sendTo will be a list, so you have to loop over the list and add each address one at a time in mail.
there's different kinds of recipients. You need to use a "to recipient".
Try this:
tell application "Contacts"
set sendToList to value of every person's email 1
end tell
set emailSender to "me@me.com"
set theSubject to "The subject of the mail"
set theContent to "message body"
tell application "Mail"
set mesg to make new outgoing message with properties {sender:emailSender, subject:theSubject, content:theContent, visible:true}
tell mesg
repeat with i from 1 to count of sendToList
set thisEmail to item i of sendToList
if thisEmail is not missing value then
make new to recipient at end of to recipients with properties {address:thisEmail}
end if
end repeat
--send
end tell
end tell