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 web site hosted elsewhere where it is since… 1995, but to move / manage the mail for 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” to host from DH web panel, then I created on the panel all the email addresses currently in use (only six addresses)

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

www.mailboxes 1200 IN A
www.webmail 1200 IN A
mailboxes 1200 IN A
webmail 1200 IN A
@ 1200 IN MX 0
@ 1200 IN MX 0
www 1200 IN A
ftp 1200 IN A
@ 1200 IN A

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 and get the mail on the new server, but some time later my browser was unable to find, 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 ?