I am getting mysterious mail errors.

Code:
Apply Filter: Only show the error messages of this member Loner
Apply Filter: Only show the error messages of this IP address 66.249.xx
  	Reverse chronological order of list Today at 02:21:27 PM
Apply Filter: Only show the error messages of this session 6e3dba062edf69683840783792eb11b4
Apply Filter: Only show the errors of this type Type of error: General
Apply Filter: Only show the error messages of this URL
http://warriorcatsrpg.com/index.php?board=646.216;wap2=
Apply Filter: Only show the errors with the same message
Ran into problems sending Mail. Error: 501 <daddy'sgirl2049@earthlink.net>: "@" or "." expected after "daddy'"
Obviously this is an invalid email address so the person isn't receiving anything.

This email does belong to an account on my site. However...

they haven't logged on since last october

this error was generated by a "loner" {guest} the guest is Google.

the member in question is not set up to receive any notifications by email at all.

The other set of mail errors I get:

Code:
Loner
Apply Filter: Only show the error messages of this IP address 66.249.xx
  	Reverse chronological order of list Today at 02:19:19 PM
Apply Filter: Only show the error messages of this session 0b130ff9677ab72455fac31e9e14ea6a
Apply Filter: Only show the errors of this type Type of error: General
Apply Filter: Only show the error messages of this URL
http://warriorcatsrpg.com/thunderclan-territories/mossleaps-kitting-private-33551.40.html.
Apply Filter: Only show the errors with the same message
Ran into problems sending Mail. Error: 501 <>: missing or malformed local part
Again--Googlebot visits are generating these. I do not know why. I have already asked my host. They don't know.

I had a slight email changeup done back last year that was sort of an informal coding tweak. I got help for altering the code so that guests do not need to input an email, in fact the email field is not there for them. Would this have anything to do with it?

How would I go about fixing these?