It isn't the number of accounts but the number of profiles...  Multiple accounts can be incorporated into the same profile.  Yesterday I configured a laptop that is attached to our domain network and concurrently has several drive mappings to a Windows 10 desktop with SSD drives we have set up[ for secure remote access through a VPN.  The laptop is using ProSeries 2014 through 2019 in a network configuration.  The laptop is using MS Office 365 Business Premium through our registered domain name.  We have no issues emailing directly from ProSeries, or printing, or printing to PDF (but we have a full Adobe Acrobat Pro implementation.

We did not need to check the 64 bit box for the email client and I suspect, (but can not prove), that because it is a network installation and the actual client files are located on the Windows 10 computer that is acting as a peer to peer server, the issue with the 64 bit client does not exist.  We did not even check the 64 bit client on this particular laptop and it just works with no errors.

There are a great many variables in each technical environment from hardware, operating systems, horizontal and vertical software packages, security settings and configuration of routers, managed switches, and ISP internet device specifics.   It isn't the "name on the box" but the detail level configuration of each device and each software package that usually determines how efficiently all features work.

Sometimes just cleaning cache files and installing updates that Microsoft determines to be "optional" helps.  Sometimes changing a graphics drive helps.  Sometimes a permission needs to be changed within Outlook?  We utilize exchange email accounts (we have not used pop email in more than a decade).