How to configure a domain for hosting mail only at DH

Hi from France and sorry for my English... I am slowly moving some domains to DH. For now I want to keep mydomain.com web site hosted elsewhere where it is since.. 1995, but to move / manage the mail for mydomain.com at dreamhost.
It looks like it work... more or less, so I wonder if I did it right... Here is what I did :

I have added a "new" mydomain.com to host from DH web panel, then I created on the panel all the email addresses currently in use @mydomain.com (only six addresses)

I can customize the DNS entries at my registrar for mydomain.com, so I did.. now it looks like :

www.mailboxes 1200 IN A 208.97.187.203
www.webmail 1200 IN A 208.97.187.139
mailboxes 1200 IN A 208.97.187.203
webmail 1200 IN A 208.97.187.139
@ 1200 IN MX 0 mx2.balanced.looney.mail.dreamhost.com.
@ 1200 IN MX 0 mx1.balanced.looney.mail.dreamhost.com.
www 1200 IN A xxx.xxx.xxx.xxx
ftp 1200 IN A xxx.xxx.xxx.xxx
@ 1200 IN A xxx.xxx.xxx.xxx

1200 is a short delay and I will change it when it works. The xxx...xx are of course the IP for the web hosting, and the other entries are what I copied from the DH panel for that domain mail...

Now here is why it work "more or less" : on the same computer, same ISP, same session, I was able to post to an address @mydomain, access the webmail.mydomain.com and get the mail on the new server, *but* some time later my browser was unable to find webmail.mydomain.com, and my email client was getting the mail from the *former* server...
Any one could help me to understand what's going on ?

I have of course, some A name and NS entries for the domain at DH, but they shouldn't not get in the way, or am I wrong ?