Errors When Sending to a Group of Email Addresses

For several years I have been sending periodic email messages to a group of about 160 members of an organization to which I belong. I did this using Windows Live Mail until this past year, when I switched my email program to eM Client. This process has worked fine until the past week or so. I am now getting error notices like the one below after attempting to send such messages to the group. My own email address is a member of the group, so I also receive back the message I send. Since this problem began, I have not received my own copy of the message, so I’m wondering if anyone received the message or if it was even sent. My primary question is this: Are the “too long without DATA command (closing session)” and “Unexpected end of stream.” errors being detected by eM Client or by my Internet email provider? Note: Actual user names in email addresses have been replaced by xxx.

     Your message did not reach some or all of the intended recipients.

     The following recipient(s) cannot be reached:
     ‘xxx@uplink.net’ - too long without DATA command (closing session)
     ‘xxx@hotmail.com’ - Unexpected end of stream.'xxx@cox.net’ - Unexpected end of stream.