You need to modify the RSreportserver.config file to have each external domain permitted...
You won't need to restart reporting services.
I'm trying to work around the issue of permitting all domains, but haven't got it to work yet.
I have tried different combinations like filling in defaulthostname, removing hostname, removing permittedhosts. I haven't tried a wild card yet in HostName. If anybody finds out, please let me know...I'm using a remote smtp server.