Using Microsoft smtp.office365.com Server, Send fails to all accounts at Comcast.net Using Thunderbird, When I send to someone@comcast.net, I get an error back after about 24 hours. Your message couldn't be delivered. Despite repeated attempts to contact the recipient's email system it didn't respond. If I send a message using a Web based service or using my Cell Phone, there is no problem with Comcast. Info from my Sys Admin: 2 messages that you sent that are listed as “Pending”. Here is the information on the reason: Reason: [{LED=421 4.2.0 service unavailable};{MSG=}