Thank you for the reply.?
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I believe I mischaracterized the problem in my original post.
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I don't think the messages are bouncing yet.?
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It appears from the Groups.io delivery history display that Groups.io is attempting delivery, but Fastmail is temporarily refusing to accept the mail from Groups.io, with RC 421.? I have opened a ticket with Fastmail support to ask them to remove the block before the 24-hour block period expires on its own.? (Fastmail had some problems over the last few days, resulting in longer-than-usual delivery queues.)? I suspect that many Fastmail users are affected by this problem.?
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If Groups.io has a channel to Fastmail admins, perhaps their admin-to-admin request to unblock would carry more weight than mine.
