R6 SMTP with R4 clients?

Has anyone had any experience running R6 SMTP servers while the client base was still on R4? Any problems that have come up in that configuration, particularly message conversion?

Subject: R6 SMTP with R4 clients?

We ran a 4.67 client on a 6.51 server just until a month ago without special problems. The mail database for this client was 4.67 template based. No special settings for SMTP. No conversion problems noticed.

Regards, Dirk