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Postfix: Mail servers of certain providers are unable to send mail to my Postfix server / insufficient security / SSL alert number 71

I found the solution. It seems that web.de (and gmx.de) only supports TLSA certificate usage 2 (DANE-TA) and 3 (DANE-EE), see RFC 6698, sec. 2.1.1. Hence, the TLSA record must be letsencrypt._dane.my-...
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