If you need Inbound mails I'd definitely recommend looking at both Sendgrid and Mailgun (and Mailjet if they offer this now), then decide on what suits your particular needs more.
I will try to write those down for the future and share back.
It's mostly via twitter that I see those, but here are examples this week:
- http://www.rudebaguette.com/2012/11/26/teleportd-nitrogram-i... ($1M)
- http://www.pehub.com/174632/youscribe-raises-e2-million/ (€2M)
- http://frenchweb.fr/la-ruche-qui-dit-oui-leve-15-million-eur... (€1.5M)
- http://www.rudebaguette.com/2012/10/11/iadvize/ (€1M)
- http://www.rudebaguette.com/2012/10/17/clever-cloud-launch/ (amount not disclosed)
> At the end of May 2010, Julien Tartarin and Wilfried Durand meet...
> We pull all-nighters and the product takes shape. A bêta version is launched after Le Web 2010. The first version goes live in February 2011. One year after, Mailjet gathers more thant 15 collaborators and sends tens of millions each week!
Also, check their job offers page 
I'd say they're already on cruise mode, and gathering people and funds to grow. Here investments of such scale are reserved to low risk, well established start-ups.
Apparently, eFounders (that participated in this round) is their incubator where they are building other French startups.
My mailjet wish list:
- fully implemented transactional email reporting (currently campaign only)
- email receipt and delivery acknowledgement on their event api (currently only reporting bounces, opens, and clicks )
- better helpdesk/support interface ( no way to read closed tickets, no defined process for escalating or closing tickets)
- more responsive support ( e.g. same day response for 'critical' tickets would be nice )
So imagine my surprise that someone is raising millions of dollars to have another whack at it.
It's always good to be reminded that solving big, new problems isn't necessarily more important to people than solving small, existing problems better.
So many. However, it's a very large market that is still seeing plenty of growth, and being able to reach your captive audience reliably and consistently is still a problem, especially when your customers may be in different parts of the world.
Deliverability is a different case in other parts of the world, funny enough. Different gatekeepers and even processes sometimes.
Does what it says on the tin! Was setup in minutes, the helpful page on DKIM and SPF was a nice touch, now we're working through the email backlog.