Due to problems with the spam filter service Spamcop, there have been and still are numerous mail blocks worldwide since yesterday.

UPDATE Feb. 3, 8:15 a.m.:
Spam filtering via the Spamcop lists has been activated again

UPDATE 12:40pm:
Spam filtering via the Spamcop lists has been temporarily deactivated
(see also circular mail from 12:38 p.m.)

See also:
https://www.heise.de/news/Spamcop-verbaselt-seinen-Domainnamen-5042358.html
https://www.sueddeutsche.de/wissen/technik-darum-kam-ihre-e-mail-nicht-durch-dpa.urn-newsml-dpa-com-20090101-210201-99-255534
https://www.fr.de/wissen/darum-kam-ihre-mail-nicht-durch-zr-90187597.html

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