[Mailman-Users] Users complain, they don't receive mails from the list

Hello,

I have some user on my lists, who complain they don't receive the list mails anymore, since a few days.

Am am running Mailman 2.1.15 on Debian. The error log is empty, the archive seems to correctly contain all recent mails. The people saying, they don't receive the mails are on GMail and German Web.de hosts and they don't have bounces logged by Mailman.

Can you help me with this? Are there any means, how I can analyse this issue?

Kind regards, Sascha.