Searching a string in a list in python

If speed is really an issue for you, you can look into methods like Aho-Corasick. There are plenty of implementations available, like esmre/esm http://code.google.com/p/esmre/

As pointed out by @Riccardo Galli, simple string matching will produce some false positives, so you can try with esmre first, adding according regexes into index, something like index.enter("(^|\.){0}$".format(domain))

Opposite to other answers, here 'foo.com' would not match also '@y.afoo.com'

def mailInDomains(mail,domains):

    for domain in domainList:
        dLen = len(domain)
        if mail[-dLen:]==domain and mail[-dLen-1] in ('.','@'):
            return True

    return False

First, make domainlist a set. It will be faster to check whether there is something contained in it.

Second, add all 'superdomains' into this set, such as 'bar.com' for 'foo.bar.com'.

domainlist = ['foo.bar.com', 'bar2.com', 'foo3.bar3.foobar.com']
domainset = set()
for domain in domainlist:
    parts = domain.split('.')
    domainset.update('.'.join(parts[i:]) for i in xrange(len(parts)-1))

#domainset is now:
set(['bar.com',
     'bar2.com',
     'bar3.foobar.com',
     'foo.bar.com',
     'foo3.bar3.foobar.com',
     'foobar.com'])

And now you can test

if tokens[1].partition("@")[2] in domainset:

Hundreds of mail addresses should not be an issue. The following is a one-liner:

any(domain.endswith(d) for d in MY_DOMAINS)

Here, you can do user,sep,domain = address.rpartition('@'). Otherwise, your current method will fail for email addresses such as "B@tm4n"@something.com, which are valid according to https://www.rfc-editor.org/rfc/rfc5322

If performance becomes a factor, you can use a Trie (a kind of data structure). If performance is still a factor, you can use other tricks.

The above goes through each element in the domains you're checking, so if you have 1000 domains in your list, you need to do 1000 lookups for each email address. If this is an issue, you can do this to achieve O(1) per lookup (you also probably want to make sure you're not checking more than 5 suffixes, to protect yourself from maliciously crafted email addresses).

MY_DOMAINS = set(MY_DOMAINS)

def suffixes(domain):
    """
        suffixes('foo.bar.com') -yields-> ['foo.bar.com', 'bar.com', 'com']
    """
    while True:
        yield domain
        parts = domain.split('.',1)
        if len(parts>1)
            domain = parts[1]
        else:
            break
def isInList(address):
    user,sep,domain = address.rpartition('@')
    return any(suffix in MY_DOMAINS for suffix in suffixes(domain))