Spam protection has matured somewhat so that around 70% of spam is automatically blocked on our standard services and up to 95% on our premium services and this is mainly due to realtime blacklists, reputation, netblock flagging, and sender frameworks like SPF or DKIM. There are however some 'providers' who are almost permanently on blacklists and who seem to take no action leaving their customers with endless problems getting email's delivered.

I'm going to highlight a few of these now just for reference but be aware this isn't an exhaustive list.

Google
For all the good Google does to the internet ecosystem, their mail service is almost permanently blacklisted. This seems to be due to the fact that their 'business' customers share the same mail gateways as their gmail customers and since gmail still factors in a significant number of spam reports its going to be blacklisted. For this reason alone I'd be cautious before using Google for mail.

Mimecast
Another name that is regularly featured in spam reports and complaints of non-delivery from customers trying to send via mimecast. Like Google this provider seems to be almost permanently blacklisted. I'm not sure what markets they operate in but another one to be cautious about for business use.

Known Spammers
There are a number of domains that are permanently on sender blacklists, its even hard coded into some distributions. I'll list a few here...

cmail.com (and cmail1.com, cmail2.com etc)
safe-sender.net
sendgrid.net (and all its subdomains)
rsys5.com
b2b-data.co.uk
jelbo.com
maildroid.hu
shot-ems41.co.uk
myspamonline.com (and subdomains)
myexchangeonline.co.uk
yandex.net (.com / .ru)
mailjet.com
dnccmail.co.uk (and subdomains)
onedirect.co.uk (and subdomains)
acemsrvg.com (and subdomains)
mhmail.co.uk (and subdomains)
dotmailer.com (and subdomains)
mailengine1.com (and subdomains)
exacttarget.com (and subdomains)
dealchecker.co.uk
listinfous.com
friendllp.com
yousave.email
mkt5663.com (and any variant of mkt with some numbers and .com)
sendpulse.net
mail battery.com
sendnice.com
infra scale.com
msgfocus.com
newzap.co.uk
createsend.com
kundenserver.de
mlsend1.com (and mlsend2.com, mlsend3.com etc. )
amazonses.com (almost always blacklisted)
msgapp.com
mktomail.com
pur3.net
kingdom.net
salescatalysts.com
wrm6.com
e-comms.net
constantcontact.com
topica.com (and topica*.*)
mkt3148.com (any variant of mkt followed by numbers followed by .)
elasticemail.com
sendmachine.info

and those are just the ones that come to mind. There are a virtually endless list of these and its updated hourly as spam reports come in and are investigated. If your interested in using our sender blacklists on your mail system then let us know?

In summary, consider carefully the options before hosting your corporate email solution with a company that is regularly blacklisted like those above or you may find problems sending email's.

E&OE - Opinions expressed are that of the author and they are just opinions. Constantly moving area so information may be out of date.