Reinstalling Catalina

You can boot in Recovery Mode and then you have two options:

(1) Reinstall Catalina without reformatting the drive. This leaves all your files as they currently are.

(2) Erase the drive and install Catalina. This would mean all your files are erased and the new OS installed. This is the most "fresh" way to reinstall Catalina.

Having two good backups before doing either of these (but especially the second option) is important. Two backups, in case something goes amiss when you are working with one of the backups.

I don't expect that option 1 would affect Parallels, but in case it does, the backups are important to have. I have Parallels running on my work machine and have never had an issue with operating system upgrades, but you wouldn't even be doing an upgrade, just a reinstall of the same OS.

I would suggest trying to work directly on resolving the mail issues first, as if it is a setup problem with the accounts, reinstalling the OS would not change that. Try smaller things first before bigger things.

If you cannot send email, I suggest first looking into your mail account settings and check that the password is properly entered for outgoing mail (sending), there is often a separate place to enter that password even if it is the same password as for the receive email. I have had to do that a few times myself for a POP server we use for several email accounts. Also check that your mail setup (outgoing servers) is correct, maybe re-enter that info if need be, or even remove and reinstall the mail accounts in question. Creating good backups just prior to any of this is important, you can always restore from the backups to what you had before if the "fix" makes things worse.

Some more information about the outgoing email errors you see (messages, notifications etc.) might enable people reading this to help you resolve the email issues.

Yes and make sure it is APFS with GUID Partition