Monterey and Mail

Don't use any third party "antivirus". They offer zero protection, cause crashes and diminish performance.

Effective defenses against malware and other threats

System Preferences > Network : you should see it in the left-hand panel eg. in my case, Avast Security. You can safely delete it using the - at the bottom and re-start your Mac. If this doesn't work, you can always re-activate it.

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is desshoigned solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Also, unless you're using a true VPN tunnel, such as between you and your employer's or bank's servers, they are useless from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Former Malware Distributor Kape Technologies Now Owns ExpressVPN, CyberGhost, Private Internet Access, Zenmate, and a Collection of VPN “Review” Websites.  However, that being said one of the most respected contributors to these forums and a respected developer has reported favorable about NordVPN.  So it may be more secure than most of the other public VPNs.

Follow the developers' instructions for uninstalling any third party "antivirus". Do not reinstall.

I had the same issue but only with my iCloud email, none of which would load contents while other emails did load as normal. This was only on my Macs, not on my iPhone. I eventually discovered that the culprit was Avast Security; once I had disabled the proxy setting and re-started, hey presto back to normal. I recall that something similar happened with the upgrade to Big Sur, it took Avast a while to update their (otherwise every good) product.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. 

So paying for software to protect from a Non Existent Issue ? Money down the rabbit hope IMHO not to mention the resources used by this software.

Mail seems load fine for me but the search function doesn't work.... (it also doesn't work with Notes but works fine in Finder.

In your Antivirus/malware Just disable the mail content checking. And Contact your antivirus provider for an update or resolution. No need to uninstall the whole antivirus.

Amirali