MS-Mail

Currently we are using MS-Mail/NT 4.0 as our exclusive organizational e-mail service. Also, we have users using Outlook 97 and 2000 and the old Win ’95 messaging (“inbox”). Quite suddenly, about a month now, a number of users are complaing about receiveing e-mails the next day when they sent the mail a day previously. Has anyone experienced this? Or, any ideas what causes this?