Gets SMTP message size for the MX servers for one or more domains using the EHLO command

ehlo-size.py

## Based on
## - https://erlerobotics.gitbooks.io/erle-robotics-python-gitbook-free/smtp/getting_information_from_ehlo.html
## - https://programtalk.com/python-examples/dns.resolver.query/

## Example usage:
##   python ehlo-size.py google.com outlook.com
##   python ehlo-size.py aol.com

## Requires:
##   pip install dnspython

## On Mac OS X, do not use `brew install pip` as then you have two different Python installations
## Instead:
##    sudo easy_install pip
##    sudo pip install --update pip
##    sudo pip install dnspython

import sys, smtplib, socket, dns.resolver

if len(sys.argv) < 2:
    print "usage: %s domain..." % sys.argv[0]
    sys.exit(1)

domains = sys.argv[1:]
domainsLength = len(domains)
singleDomain = domainsLength == 1

for domain in domains:

    try:
## use `answers` outside and `rdata` in the loop as that is what DNS folks call it:
## - http://www.zytrax.com/books/dns/ch15/#rdata
        answers = dns.resolver.query(domain, 'MX')
        answers = sorted(answers) ## sorts on preference; might want this more clear using https://github.com/linkedin/iris/blob/master/src/iris/vendors/iris_smtp.py#L46 or https://developers.google.com/edu/python/sorting
        for rdata in answers:

            allMxForDomainFailed = True
            try:
                mx = rdata.exchange
                priority = rdata.preference
                server = mx.to_text() ## to prevent 'Name' object has no attribute 'find', see https://stackoverflow.com/questions/48071359/why-do-mxrecord-should-be-change-to-string
                s = smtplib.SMTP(server)
                code = s.ehlo()[0]
                uses_esmtp = (200 <= code <= 299)
                if not uses_esmtp:
                    code = s.helo()[0]
                    if not (200 <= code <= 299):
                        print "Remote server refused HELO; code:", code
                        sys.exit(1)

                if uses_esmtp and s.has_extn('size'):
                    maximumMessageSize = s.esmtp_features['size']
                    if maximumMessageSize == '':
                        print("Undefined aximum message size for domain {0} MX priority {1} SMTP server {2}".format(domain, priority, server))
                    else:
                        print("Maximum message size is {0} bytes for domain {1} MX priority {2} SMTP server {3}".format(maximumMessageSize, domain, priority, server))
                else:
                    print("No defined maximum message size for domain {0} MX priority {1} SMTP server {2}".format(domain, priority, server))

                allMxForDomainFailed = False
            except (socket.gaierror, socket.error, socket.herror,
                    smtplib.SMTPException), e:
                print("Error when executing EHLO command for domain {0} on SMTP server {1}".format(domain, server))
                print e
                if singleDomain:
                    sys.exit(2)

        if allMxForDomainFailed:
            print("All MX for domain {0} failed.".format(domain))
            sys.exit(1)

    except Exception as e:
        print("Error when querying MX records to obtain SMTP servers for domain {0}".format(domain))
        print e
        if singleDomain:
            sys.exit(4)