Can you use postfix with horde/imp?
Yes, Horde doesn't care what the SMTP provider is so long as it works.
Well, postfix is just a MTA. IMP will use localhost:25 or /usr/lib/sendmail to send mail ;-) What's more interesting is the choice of IMAP-server ;-)
Yep.
We currently use dovecot. Any issues that you know of?
No issues.
Hm. I use dovecot, too, on my private mailserver - but I don't have a webmail setup there, because I use IMAP exclusively. I always wanted to try IMP/DIMP again, but never really got around to install and configure it.
We use Horde/IMP (and testing DIMP) to provide web mail to our low-end users. Horde is great stuff, works across browsers well, is stable, and deals with every tortured mail message (including ones that wig out either Thunderbird [not really very hard to do :(] or Outlook]. For groupware we use OpenGroupware, and Horde is very easy to integrate into almost anything.
I know one ISP here, that tried to use IMP and it performed very bad.
They set it up wrong.