I have tried to send a document to certain SAP user by using the send to option.
However, the send to page prompt me a "Mail could not be sent" error.
The email channel setting are configurated correctly because the notification and workflow email are working fine.
Thank in advance
Even though your notification emails are working that doesn't mean that all email channels are correctly configured as send to uses a different email channel.
Subscription notifications use the 'EMAIL' channel while send to functionality uses the 'INTERNET' channel so you have to make sure both are correctly configured. Check SAP note 808756 for more details.
Please check the defaultTrace..trc file in the path /usr/sap/SID/JC0X/j2ee/cluster/server0/log for more details. Please paste the trace file log here if possible.
Also, check whether there are an anti virus software/firewalls in your system which could prevent the mail being sent.
Thanks and Regards,