''<p>robi99 [[#question-1508890|said]]</p>'' <blockquote>A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: andrew.krischock@outlook.com host outlook-com.olc.protection.outlook.com [52.101.40.5] SMTP error from remote mail server after pipelined sending data block: 550 5.7.1 Unfortunately, messages from [103.150.252.1] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.asp[truncated] </blockquote> So what does you mail provider (atmailcloud) have to say about the block listing of the mail you send? The error is pretty clear the issue is with the server and Thunderbird doe not supply a server, your mail provider does. atmailcloud is the base domain for the IP address mentioned. I have no idea who they are, but it appear they are https://www.atmail.com/ or Atmail Pty Ltd Suite 251, Level 18, 324 Queen Street, Brisbane QLD, 4000 and they appear to be linked in someway to "The Messaging Company" which at least one major Australian ISP passed all of their email services to in the last 12 months. SO perhaps you need to contact them, not your ISP.