As the attachement(.zip) less than 10MB is sent successfully, its clear that the restriction on the size.
This 10MB is the default limit set by most of the Mail hosts, just to have a threshold on the spamming of their disk space.
The mail which you sent to Mail host is successfull technically. But as the host rejected its not further sent to the mail box.
You can change the setting of this 10MB in the SMTP server. Its a configurable setting. There were cases where ~25MB files are sent across mails.
#webMethods#webmethods-Protocol-and-Transport#Integration-Server-and-ESB