It could be several things. Is it POP mail? A setting in the account in email client (Outlook? Thunderbird? or ?) Or it could be her virus scanning interrupting the down/receive - the mail scanning feature. It could be a corrupt file in mail client (the file that keeps a record of which have already been downloaded)
Is it only from these six senders or is it all of emails?
I suggest as a short term remedy to login to Webmail server and tidy folders up there deleting emails not needed any more.