As a sender of millions of B2C 3rd party emails a month, the biggest problem with deliverability that I come across other than the ISP issues already discussed, is designers!! Every day I get creatives sent to me from designers who have failed to take into consideration the key issues for deliverability and SPAM. The main problem I encounter frequently relate to not having designed the email for preview panes so all the recipient sees in a preview pane is a block of grey boxes where in the majority of cases images have been blocked by the email providers!
The main issue with email deliverability is that it is a variant of so many factors that a lot of organisations don’t seem to understand; namely that in order to achieve a high rate of deliverability you need to take into account not only how the email is seen when it reaches the inbox (hopefully anyway :-) ) but also issues relating to Authentication and Reputation of the sender which have become increasingly important issues in this area over recent years, as well as designing the creative to get through the spam filters. In relation to the designers that I encounter, they seem more preoccupied with the 'fanciness' of the email - it doesn't matter how fancy and brilliant your email is if you cant get it into the inbox successfully!! Saying that though I do admire the creatives they design (especially when they are SPAM and deliverability friendly!)
