A universal email crawler
There are several ways we can go to make the code cleaner and faster. Grouping by the scope.

search_item() function


link would probably be a better variable name than a less descriptive item
move the substrings you are looking for into a constant - defined as a list of keywords
then, you can use any() instead of multiple or conditions
fromstring() returns a root node of the tree and not the tree itself - would probably be more logical to name the variable root instead of tree
do not re-do item.lower() - do it once


You may get something like this in the end:

KEYWORDS = ["contact us", "about us", "contact", "about"]

def search_item(driver,url):
    driver.get(url)
    root = fromstring(driver.page_source)

    for link in root.xpath("//a/@href"):
        link = link.lower()
        if any(keyword in link for keyword in KEYWORDS):
            get_item(urljoin(url, link))
            break


get_item() function


you should pre-compile your regular expression
use .search() instead of .findall(), since you are looking for a single match
this would then allow you to avoid handling a bare exception and switch to LBYL approach instead of the EAFP


You may get something like:

EMAIL_PATTERN = re.compile(r'[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+')

def get_item(link):
    response = requests.get(link).text

    match = EMAIL_PATTERN.search(response)
    if match:
        print(link, match.group(0))


Few other general comments


not sure if "contact us" and "about us" (with a space) would ever appear in an href value..
re-group your imports as per PEP8 recommendations and avoid having multiple different imports on the same line
main_url should probably named be something like start_urls (you need a multiple of "url" since you have a list of them). And, you need to indent the items since you are defining your list on multiple lines:

start_urls = [
    'http://advanta.in/',
    'http://warnaengineeringworks.com/',
    'http://unifrostindia.com/',
    'http://www.superrefrigerations.com/',
    'http://www.westernequipments.com/',
]

consider adding documentation strings to your functions