But you have some things missing from your calculation:
1) Attachments (yes, SES charges separately)
2) Dedicated IP addresses (SES charges you separately for this)
3) S3 (where emails are stored
4) AWS Lambda (because you need a script/function, that processes incoming messages as they come in).
But yes, you're right, with SES its cheaper than others, couple of thousand tops
> costs a lot more
The point was super clear, and yet you managed to miss it.
@jitbit clearly stated that he and his colleagues evaluated several possibilities, and the decided to set up their own system.
It's literally short-term decision: it's a make-or-buy problem.
The make options surely takes some time, but it is a one-time expense with pretty much low maintenance and super-low operating-cost ($4/month). It also requires some study but hey, that's know-how that is going to stay in the company.
The buy options is a lot more costly, but gives the gift of ignorance: you are not required to know or do anything.
And if you are wondering what the costs are: setting up a basic mail server for a domain takes as little as a couple of hours. A little-more complicated might take a day, and a complex setup not more than a week, for a skilled person.
Considering other options, it might just be cost-effective to hire a consultant to set it up.
Source: i've been running my mailserver for years, and I've done consulting in setting up and troubleshooting mail servers.