Cannot Send Group E-Mails

For some reason, I just stopped being able to send Group e-mails to four groups with about 50 e-mails in each group.  I only do this two or three times per year and for some reason it stopped working.  I get the message "The message could not be sent.  The setting for your outgoing e-mail (SMTP) server might need to be configured.  To find the Server settings for *** Email address is removed for privacy *** please contact your e-mail server provider."  I did this and Comcast does not know what is wrong with my settings and they asked me to contact Windows Live Mail at 800-728-8890 which does not answer - apparently during the Christmas holidays.  So, I thought I would try this forum and it seems that Microsoft is the problem here according to what I have been reading.  I need to send a message to these groups and am now at a stand-still.  I have been sending to the groups for several years without problems unless an e-mail address was incorrect, and then would receive the name of the address that was not valid which made it easy to either delete it or correct it.  Where do I go now to get an answer or has Microsoft resolved the problem in another version of Windows Live Mail.  I use Windows 7........Leon Ullrich

Added - I am getting a Server Error of 550. a Server Response of 550 5.1.1 not our customer

Also Server: "smtp.Comcast.net" and Windows Live Mail Error ID of 0x800CCC79 with Protocol of SMTP and Port of 465 which was set by Comcast.  Comcast seems to be unable to help and referred me to Windows Live Mail and as I indicated does not answer the above number.  Is there another?  In addition, the group I am trying to send has 50 email addresses in it, but I am able to send a Group with 7 email addresses in it without any problem.