Larger (all?) email providers treat these as the same address:
first_name.last_name@domain.com
first_namelast_name@domain.com (no period between first and last name)
Mail.app smart mailboxes also seem to treat the above recipient addresses as the same.
So, what AppleScript would find Mail.app mails with recipients whose address does not contain the dot?
If the first name and the last name are not separated by anything, then the computer cannot guess which part of the sentence is the first name. In this case, you need to help it by providing it with a list of possible names in advance. For example, a list of American names. Here’s an example:
property predefinedFirstNames : {"James", "Bruce", "First_name"}
set email to "first_namelast_name@domain.com"
ignoring case
repeat with nextName in predefinedFirstNames
if email contains nextName then return nextName
end repeat
end ignoring
Thank you for your guidance, KniazidisR.