Something that will help end-users troubleshoot why their signature can not be confirmed. By programming more intelligence when signing a petition with a known problematic e-mailaddress. For a week, or even shorter, the bounce message of the first attempt is stored and the relevant lines are shown (preferably with a summary in non-technical terms) when someone tries to sign a second time with that address. Errors like user does not exist (typo before the @?), over quota (please delete some mail from your mailbox) or some auto-reply. The error could be stored in the database in the signatures table in an extra field.
The errors should also be visible to the petitioner who can then lower the percentage of unconfirmed signatures. By fixing common mistakes in the e-mailaddres (gmail.nl instead of gmail.com for example) or resending a timed confirmationmail after the person's holiday or a customised confirmationmail saying that one can not reply with 'yes, I sign' but should follow the link in the confirmationmail (which should be intercepted, otherwise the petitioner is clicking on behalf of the signatory).
Signatories who have signed and confirmed and get the update e-mail (voortgangsbericht), the message that comes back is stored with their signature. The petitioner can do maintenance on this. The auto-reply with a new e-mailaddress can be used by the petitioner to urge to sign again with the new address. A confirmation e-mail with the new address goes out, explaining that the old signature will be deleted and the new will replace the old. This should be a one-click effort by the petitioner when going through the bounces.
The intelligence on what are misspelled e-mailadresses (like gmail.nl) can also be employed for the invite to sign-functionality. The known problematic adresses will be shown and asked to correct into what is most often the problem: firstname.lastname@example.org is not a correct address, do you want to change it into email@example.com? yes/no
See the mailbox for firstname.lastname@example.org to study what bounces come in now.