Can't connect to SMTP server

Can't connect to SMTP server

Hi,
i'm trying to send mail from a mail client through Zoho's SMTP server, however the connection keeps on failing.
The error message from Thunderbird is:
"Sending of message failed.
The message could not be sent because the connection to SMTP server smtp.zoho.com timed out.
Try again or contact your network administrator."
Other mail clients (Opera Mail, Windows Mail, Android Mail) give similar error messages or simply refuse to send any mail, keeping it in the outbox forever. IMAP access and sending mail through the web interface works flawlessly. I don't have any such problems with my other mail provider either. The settings are as recommended (smtp.zoho.com:465 with SSL).
Any ideas what could be causing the problem?
Kind regards,
Yannick