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Thank you for your help which has worked. I have two further queries on this issue:
When I login and there is a message waiting for me, an error message appears "Could not instantiate mail function" even though the message has been received and I can view the body of the message? How can I get rid of this error message?
I can not only see a "1" against the mail icon in the header but there is also one against the notification icon?
Jomsocial is not execute sending email once you send the message, but jomsocial will save the email to the emailq,and then it will be executed via cron job
Actaully issue caused by your mail server config, please you check your Joomla Global Configuration and check your "Mail Settings", for further information I suggest you post to Joomla forum.