Hi and welcome to the forum
Most things you add or configure through the panel will have some sort of delay. The delay period will vary depending on what you are doing, but generally you will have to wait 10 or 15 minutes for most changes to take effect.
I am not sure if you mean you want to point to external DNS servers somewhere or you want to use the DreamHost DNS servers but have them shown as ns1.yourdomain.com etc. Both of these options are possible at DreamHost, but the second one does take a little extra configuring.
There is no uptime guarantee and I can't really give you any concrete uptime figures as uptime depends a lot on the individual server you end up on. All I can say is that my sites have generally had very good uptime since I have been here at DreamHost. It hasn't been perfect, but outages are not too frequent and usually of very short duration. On the rare occasions that I notice my sites are down, they are usually back up before I have a chance to lodge a support request.
Webmail is provided via SquirrelMail and is accessed through webmail.yourdomain.com
I believe the limits are 100 per hour for SMTP and 200 per hour for scripts/shell etc. on a per domain basis, but you are advised to ask DreamHost this question directly for the definitive answer. I don't think we have access to raw mail logs, but once again, it would be best to ask DreamHost this question.
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