The end of file was reached error Outlook for Mac 2011

Hello, I'm running Outlook for Mac 2011 14.7.7 on an old iMac running OS 10.13.6.

I have several email accounts set up within OL all using POP3. Most are functioning fine, but when I try to send mail from my primary address I get the "The end of file was reached" error.

I have tried several things so far:

I have rebuilt the Main Identity - problem persists.

I have created a new identity and tried sending from that - it worked. But for some reason I was not able to import my .olm into that identity.

I have copied out the MUD folder and deleted the original and then relaunched OL from a Time Machine backup. I think at the point it may have been sending without the error? But I then imported the .olm to replace some missing data (and then deleted some duplicate data). The error is back.

I've made extra space on my HD and deleted some emails to try to unclog things a bit.

Any suggestions are very welcome!