A MTA (also known as Mail Transfer Agent or Mail Relay) is software that sends emails from one a server. It can both send and receive email messages. MTAs have complex configuration options, allowing a server administrator to customize nearly every aspect of email delivery.
Anyone who is sending significant volumes of emails should seriously consider using a commercial MTA product, primarily because of all the "what ifs" that can and quite often do happen. You want to be covered should something break.
Some of the features of MTAs include:
Great question. Without the right hardware and a reliable host, all the effort you put into configuring a MTA could be in vain. Most cloud environments are not up-to-snuff for high-volume MTAs, because you really need a fast CPU and drive(s). You're also risking your email reputation being effected by other customers sharing your IP or subnet, and that's a risk you don't want to take.
I recommend a dedicated server with SSD drive(s), multiple-cores, and decent RAM (4 GB+). You could probably get by with less, but that's just my personal preference.
As far as your host or datacenter goes, it's important that they have a good reputation with the anti-abuse community. Ideally they are also proactive on any of their other customers who are accused of sending spam, in order to protect the reputation of everyone else.
Finally, I don't want to be Captain Obvious, but it's crucial to secure your system properly. Some things to consider include:
Some of the more popular MTAs include:
Kane Miller from Winning.Email
Kane has a background in computer engineering, but now focuses on creating new and interesting web services. His passion for improving his own email deliverability rates was the driving force behind the creation of Winning.Email.