Can't send but can receive e-amil

Can't send but can receive e-amil

When I try to send an e-mail via my zoho account I get an error message with "Reason: eof". I can recieve e-mail without a problem.
Thank you for your help.

Update:
If I try to send a mail from the Dashboard I get this message:

Attachment error

Unable to send message;Reason:[EOF].