[igor_a]

I haven't worked with external emails providers. For now we are sending around 100-150k emails/day and we are using postfix as MTA.

Regarding server side rejects - again reading postfix logs can save you from such things. As an example recently we started seeing that we can't deliver a lot of emails to orange.fr, and the worst - if send just one email it always was accepted but when we sent bulk newsletter we had a lot of rejections - the reason was quite simple we've got rejected because of high frequency so we just had to throttle postfix on per domain basis.

[edgesrazor]

I think I might have to set up postfix and do some experimenting. One of the strangest things I've seen is that some domains are rejecting emails with links to our site in them, but will accept them if the links are removed (plain text or HTML).

I swear, someone could make a good living handling this for companies looking to handle transactional email internally.