My intent is to migrate an existing FC2 server with web and email to Centos 5.5 on a linode.
The existing server is running the following:
DirectAdmin
exim 4, clamav, spamassassin
uw imap
apf firewall
apache,perl,php

Mail access is via imap only, via SSH tunneling.

The exim routing (setup by DA) is based on configuration files stored under /etc/virtual
/etc/virtual/domains - has a list of virtual domains
/etc/virtual/$domain/ - holds several domain configuration files:
aliases - holds all aliases for $domain
whitelist
blacklist
filter
...
The incoming mail is currently stored in mailboxes:
/var/spool/mail/$user
Archived mail is stored under:
/home/$user/mail/(whatever)

The aliasing structure is such that mail addressed to various email addresses in different virtual domains may all end up in the same mailbox. Conversely, mail sent to a single email address may be distributed to several mailboxes, after 3 or 4 levels of aliasing.

I have Centos 5.5 i386 installed on a 2.18 stable kernel. (iptables had problems with later kernels. This one seems to be fine) I have a sandbox domain for testing. I installed Webmin to use for a panel. I installed csf, as Webmin is friendlier to that than to apf, and seems to have some other nice features.

Plan A is to make exim run on the new server, as it does on the old server. I installed exim, but am having trouble getting it configured to behave as it does on the DA platform. (It works in debug mode on emails entered from the command line but rejects emails sent over the internet. I am beginning to suspect the problem may be related to the fact I am using the linode name servers instead of running my own named).

CentOS (and everybody else) seem determined to move me to Postfix/Dovecot. I am willing to make the switch, IF that configuration will support the same aliasing structure, mailbox layout, mail headers, etc.. (There are several thousands of emails to migrate)

Should I try plan B (Postfix), or continue with plan A?

Any advice appreciated. I have never used Postfix, and have no idea how to configure it. But am getting frustrated with exim in the new environment.

Well, I use Postfix/Dovecot on CentOS, but given the nature of your setup I'd suggest making another effort to get Exim working correctly. There's going to be a fair bit of working migrating thousand of accounts & aliases to Postfix/Dovecot.

You say that Exim rejects mail sent over the Internet. What exactly does it log?