Can't send emails w/ attachments in Mail app

Hi,

Recently (last ~2-3wks), I have no longer been able to send any emails w/ attachments regardless of size (not even an empty plaintext file) from my mail app. When I do it hangs out in my outbox for a bit and then alerts me that "Cannot send message using the server ____", but the connection doctor reports that it can connect to all my accounts without issue; which makes sense as I can send emails without attachments hassle free.

This occurs for all of my email accounts. Ensuring that my login credentials are up to date didn't solve the problem. Online I found a few suggestions, one was to remove the accounts and re-add to Mail, this didn't fix the issue. Another was to change the authentication settings to 'password', this also didn't work. Additionally I rebooted into safe mode and opened the Mail app (this supposedly would clear the cache), still no luck. These are all the options I could find in my searching, so I'm at a loss for how to proceed from here short of updating macOS (not preferable as this is not my machine & we have in-house software that has yet to be vetted w/ later versions)

This is specific to my iMac at work (mid 2011 21.5"; El Capitan), no issue are experienced when I use my macbook or iPhone.

Any help would be greatly appreciated!

Brett