We have recently transferred our website to a new CentOS server and in the process have changed from sendmail to postfix. Emails to most destinations now seem to be working OK, but not those to btinternet.com. However, the maillog file indicates that btinternet.com emails are delivered (after being initially deferred). Here's a typical extract from maillog (line breaks have been added for clarity and names and IP addresses removed for security): Jan 4 11:26:49 serverxxx-xxx-xxx-xxx postfix/smtp: 3FE546D650: host mx1.bt.mail.yahoo.com[184.108.40.206] refused to talk to me: 421 Message from (xxx.xxx.xxx.xxx) temporarily deferred - 4.16.50. Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html Jan 4 11:26:50 serverxxx-xxx-xxx-xxx postfix/smtp: 3FE546D650: to=<firstname.lastname@example.org>, relay=mx1.bt.mail.yahoo.com[220.127.116.11]:25, delay=0.86, delays=0.01/0/0.6/0.25, dsn=2.0.0, status=sent (250 ok dirdel) Emails sent to the same address without going through the server are delivered and received OK. In both cases I am using Thunderbird to collect them from mail.btinternet.com, port 110. Any suggestions as to why I'm getting '250 ok dirdel', indicating that the emails have been accepted by mx1.bt.mail.yahoo.com, but I can't retrieve them from the BT server? PROBLEM SOLVED I had overlooked the fact that BT now don't just store emails on their server but filter them through their webmail system. All the missing emails were in the Spam folder. I've now turned off the Spamguard (Thunderbird does the job much better anyway) and the emails are coming through OK.