Emails show as sent but are not received

Apple Mail shows messages from my iCloud email accounts as sent while recipients did not receive them (these are tech literate recipients who know how to hunt down missing emails). I'm running Ventura 13.1. I've no VPN nor email tracker plug-ins. I regularly use two different WiFi networks and it seems to occur on both, but it is difficult to be certain.

This is neither a consistent nor predictable problem so difficult to troubleshoot, but I have determined it to occur with all of the following conditions and variables:

• both when I initiate the conversation and when I reply
• to numerous recipient domains (ie - @gmail, @yahoo, @stanford, etc)
• both when I send attachments and do not send attachments
• both when other recipients are cc'ed and to lone recipients
• regardless of which iCloud address I send from (@me, @Mac)

I only learn my emails are not received when I'm contacted by someone anticipating an email and wondering where it is, or when I don't receive an expected "follow up" suggestion from Apple Mail, prompting me to reach out to the recipients to learn my original email had not been received. Once, when I followed up in such manner, I cc'ed my own external account on a sent mail reply to all original recipients, had all emails then go through, at which time the prior sent emails also went through as separate sent messages. It seems random but also seems to be increasing in frequency.

I occasionally receive an error: "The attempt to send data to the server "p28-smtp.mail.me.com" failed." This occurs when I'm otherwise connected (ie - browser functioning). It seems there is no way to cure this error other than copying and pasting the content into a new email (ie - attempts to resend fail and attempts to close the email don't save to drafts.). Additionally, I will occasionally have messages show in both the sent and outbox.

Perhaps related, when I send messages from iCloud's webmail interface they don't appear in sent messages despite having been sent and received. They also don't appear in my mail apps sent folders. When I send messages from my mail apps, they do appear in my iCloud webmail sent folder.

Thanks for any insights.