Yahoo have told me (via the feedback form) that our mail server is temporarily 'deprioritized'
This means that the first few times we connect to the mail server, we get a 451: Resources Temporarily Available, pleae try again later' message (after sending the message). We then have to retry and it seems to accept the message just before its due to expire.
I have asked Yahoo what the reason would be, but they only send generic responses. We have rDNS setup, and an SPF but no luck. Anybody had any experience with this?
Here's a conversation:
220 mta155.mail.re2.yahoo.com ESMTP YSmtp service ready
250 sender <email@example.com> ok
250 recipient <firstname.lastname@example.org> ok
354 go ahead
Subject: Test from voodoo
This is atest
451 mta155.mail.re2.yahoo.com Resources temporarily unavailable. Please try again later [#4.16.5].
Connection closed by foreign host.