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OPENPGPKEY & POSTEO

First generate a public-private key-pair with gpg as usual. Next export the public key:

gpg2 -a --export --export-options export-minimal [KEY-ID] > pub.asc

Make sure you have a proper version of the dig tool (use DiG 9.10.3-P2). And your firewall rules allow global dns requests:

dig +short +vc OPENPGPKEY \
  141fa5a54515c9fbb2ff131b23fb5b51339365324ece6efe1cabae37._openpgpkey.posteo.org \
  @8.8.8.8 | sed 's/ //g' | base64 -d | gpg2

However, this request is not fully verified.

It is a good idea to publish your public key at a second place (e.g. Website, Key Server) in case your email provider publishes any rubbish.

Sources: sys4.de, ietf.org, heise.de