GMX and Web.de limit their free of charge users in IMAP sync. This occurs to the user in eMClient as if the password was wrong, but it isn’t. You can even setup a separate password for IMAP in GMX and Web.de, but this doesn’t help with restrictions of those two.
Perhaps, when setting up those email services, you could have a check mark to use a conservative polling rate or a time in minutes in-between a follow-up sync regardless of a global sync request.