In Claws Mail 3.13.2 (this is what my distribution provides, sorry), URLs like https://www.frankfurt.de/sixcms/detail.php?id=2805&_ffmpar[_id_inhalt]=54568 are not correctly detected. In this example, the URL is shortened to https://www.frankfurt.de/sixcms/detail.php?id=2805&_ffmpar You can still see the full URL, but only this part gets underlined and works as a link. If square brackets are allowed in URLs this is a bug. I noticed this behaviour with emails in my outbox and my drafts folder. Didn't check with incoming emails.
Square brackets should not be used like this in an URL. Looks like people at frankfurt.de should have a look at RFC 3986, where in section 3.2.2, it says: A host identified by an Internet Protocol literal address, version 6 [RFC3513] or later, is distinguished by enclosing the IP literal within square brackets ("[" and "]"). This is the only place where square bracket characters are allowed in the URI syntax. https://tools.ietf.org/html/rfc3986#section-3.2.2
Thank you, Andrej! Will inform the webmaster and see if he / she cares.