LMTP error: Too many concurrent deliveries for user (in reply to end of DATA command)

[Ralf Hildebrandt]

I updated dovecot today and all over a sudden I'm getting:

Mar 6 14:40:46 mail postfix/lmtp[3150]: 3kz95y3nX3zCtTS: to=<recipient at backup.invalid>, relay=127.0.0.1[private/dovecot-lmtp],
delay=88, delays=87/0.94/0.01/0.01, dsn=4.3.0, status=deferred (host 127.0.0.1[private/dovecot-lmtp] said: 451 4.3.0
<recipient at backup.invalid> Too many concurrent deliveries for user (in reply to end of DATA command))

Why is that? Which setting must I tweak?

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