i had this problem with gmail. here's how i fixed it:
calibre settings: sharing books by email: use the optional manually inputed server settings at the bottom left as follows
hostname: smtp.gmail.com
port: 465
username: [your name]@gmail.com
password:
generate a mail application password here and use it, not your normal google/gmail password. this will fix the 2 factor authorization error that is the cause of this problem.
encryption: SSL
send test email
i have not yet changed the 5 min spam delay but i used to send several books at once from this same email so will probably change that in calibre tweaks
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information for the application password setup for smtp mail was found in this article:
https://kinsta.com/blog/gmail-smtp-server/