Receiving email with attachment

Hello - recently my company has been having trouble downloading emails into our mail client (Outlook) that have attachments greater than 4MBs. We have some people set up as IMAP in Outlook and if one of these emails are sitting in their inbox, their email will not synch or download any new emails. In the POP Email Account, it will eventually download into Outlook but it will take forever.  This isn't a speed issue, as we have very high internet UL/DL speeds. This also seems like a recent issue - never had the problem receiving emails with a semi-big attachment before. It's my understanding that Outlook will allow an attachment up to 25MBs. Does anyone have any idea what this issue is?