Outlook 2019. I am having "sync" issues and can no longer send any e-mails.

Computer crashed - new HD installed by pros. They recovered all my data from the old drive and put on the new drive.
They also Installed Office 2019, but the issue is only with Outlook 2019. It installed fine. It worked perfectly (at first). But I wanted to get my old PST files merged with the newly installed PST files. The merge seemed to work but I later realized several problems.
One: Maybe because the old PST files were so large (some over 3gb) - Outlook won't stop trying to SYNC with whatever!!! Then TWO: Now I cannot send any new e-mails!!! I read on the internet that as long as the SYNC is incomplete this stops all mail sending.
So, how can I make the SYNC complete.... or stop it altogether?
And if this is not the cause of the non-sending e-mails - what is?
I don't know what the SYNC even is or if it is that important. This is a home used computer and I use it only for e-mails and browsing, etc. Not company or big network use. Don't even know what the MS exchange is and why it needs to SYNC in the first place.
Is there a quick easy fix? Should I consider removing Outlook altogether and re-installing it? I'm open to any and all suggestions....... please. E-mail is pretty much my whole life which is why I wanted to merge the old PST files with the new install. FYI, for what it is worth, I use 5 different e-mail accounts on this Outlook.