If I send an email from the webmail, then it looks like if the mail is sent. But looking closer to the /var/log/carbonio.log I got a connection timeout.
Nov 14 13:12:23 mail postfix/smtp[1092948]: BD85250005B: to=<max@*****.nl>, relay=none, delay=3377, delays=3317/0.01/60/0, dsn=4.4.1, status=deferred (connect to mails.******.nl[185.**.**.**]:25: Connection timed out)
I tried to send to different users but for all the other mails I got the same message.
Everything looks ok. Tried to telnet to external mailservers port 25 will also get a connection timeout. Firewall looks ok.
zextras@mail:/var/log$ zmcontrol status
Host mail.********.nl
amavis Running
antispam Running
antivirus Running
directory-server Running
mailbox Running
memcached Running
mta Running
opendkim Running
proxy Running
service webapp Running
service-discover Running
stats Running
config service Running
It looks like your outbound through port 25 is restricted.
You may test using
telnet gmail-smtp-in.l.google.com 25