[qoute]a) Roundcube 0.4, which is on my hosters webspace and receives IMAP mail from the localhost, generates message-id's in the form "unique@localhost". Can I configure RC to use the domain name as the domain part of the message-id?[/quote]
You cant configure it but the message-ids have been altered in i think 0.4.1 or 0.4.2 so they use the domain rather than localhost
b) RC creates empty headers
The User-Agent one should not be empty, may be you are missing an entry in your config file? it is added in program/steps/mail/func.inc and/or program/steps/mail/sendmail.inc. The X-PGP one is not added by the RC core, do you have any plugins enabled?