What is your IP address?
Is it a dedicated or shared IP address?
How long has the IP address been sending email from you?
Do you immediately remove addresses from your list that bounce?
Are you only sending email to people that want the email?
Do you segment your lists so that the topic of the newsletters matches what the people are interested in?
Do you know what your open rates are?
It seems that how often your subscribers open the emails and click the links is starting to matter more and more.
To answer your questions above (What software do you use for sending the mailings?) :
1) Unless you are sending content that is time critical such as a coupon that expires in 24hrs to large amounts of addresses I'd think having 10 concurrent outbound connections is fine. Watch how long it takes to send and figure out if that's an acceptable amount of time.
2+3) I'm a qmail guy that sends with this
qmail retry schedule. Since you are still having problems how about no retries for now? If an email fails with a non 5xx smtp code keep them on the list but don't try to send to that address again until the next newsletter. When you have the lists and sending cleaned up then you can try adjusting to 1 retry and see what happens.
4) This is covered by 2+3 above
5) A value of 5 should be fine. Some mails servers open a connection per email. You're being nicer than that with a setting of 5.
6) I think 5 should be fine here too. You might be able to adjust answers 5 and 6 higher but if you are running in future runs I wouldn't make addition changes until there is an actual problem that you are trying to solve.
7-10) Leave them unchanged. If you continue to have problems over the next few weeks you can start to figure out which ISP's and why and try to solve specific problems. "What do the logs say?" to fix those errors.