Lua validate email address

valid-email.lua

function validemail(str)
  if str == nil or str:len() == 0 then return nil end
  if (type(str) ~= 'string') then
    error("Expected string")
    return nil
  end
  local lastAt = str:find("[^%@]+$")
  local localPart = str:sub(1, (lastAt - 2)) -- Returns the substring before '@' symbol
  local domainPart = str:sub(lastAt, #str) -- Returns the substring after '@' symbol
  -- we werent able to split the email properly
  if localPart == nil then
    return nil, "Local name is invalid"
  end

  if domainPart == nil or not domainPart:find("%.") then
    return nil, "Domain is invalid"
  end
  if string.sub(domainPart, 1, 1) == "." then
    return nil, "First character in domain cannot be a dot"
  end
  -- local part is maxed at 64 characters
  if #localPart > 64 then
    return nil, "Local name must be less than 64 characters"
  end
  -- domains are maxed at 253 characters
  if #domainPart > 253 then
    return nil, "Domain must be less than 253 characters"
  end
  -- somthing is wrong
  if lastAt >= 65 then
    return nil, "Invalid @ symbol usage"
  end
  -- quotes are only allowed at the beginning of a the local name
  local quotes = localPart:find("[\"]")
  if type(quotes) == 'number' and quotes > 1 then
    return nil, "Invalid usage of quotes"
  end
  -- no @ symbols allowed outside quotes
  if localPart:find("%@+") and quotes == nil then
    return nil, "Invalid @ symbol usage in local part"
  end
  -- no dot found in domain name
  if not domainPart:find("%.") then
    return nil, "No TLD found in domain"
  end
  -- only 1 period in succession allowed
  if domainPart:find("%.%.") then
    return nil, "Too many periods in domain"
  end
  if localPart:find("%.%.") then
    return nil, "Too many periods in local part"
  end
  -- just a general match
  if not str:match('[%w]*[%p]*%@+[%w]*[%.]?[%w]*') then
    return nil, "Email pattern test failed"
  end
  -- all our tests passed, so we are ok
  return true
end

I found another error. This string validates:
[email protected]

Add this check:
if string.sub(domainPart, 1, 1) == "." then
return false, "first character in domainPart is a dot"
end

The program will stop if string is empty ""
So, I added str:len checker:

if str:len() == 0 then return nil end

One more thing. This code will accept "@gmail.com" but it shouldn't accept it ?

Added a bunch of checks from all the suggestions. Thanks everyone