Other than that, there are no problems using round cube with safe mode.
I agree. But I think the application must not send useless e-mails, and the use of mail function in safe mode must raise an error instead of send a malformed e-mail. IMHO ;)
If you are using Sendmail you can play with the trusted users option. The php manual doesn't specifically mention that there is a restriction on the From: header in safe mode.
j