Can't send messages from desktop (Windows Vista) with wire to modem or laptop wireless (Windows 10) but can read new E-mail (Thunderbird) on both computers.
1) changed Internet modem 2016-11-12 (used by two computers) but sent messages as late as 15th on desktop (Vista & wired) but none able to be sent on 2016-11-16 or later. Receipt of error message 5.7.1 and no E-mail message added to "sent" list. 2) Similar effect (on Windows 10 laptop with wireless connection) was effective on 2016-11-16 (with no attempted sends on 14th or 15th) but message sent on 16th showed on "sent" list. Next attempt on 18th with same message (updated and forwarded message with more information), stopped with the 5.7.1. error message and did not appear on the "sent" list. 3) Both computers denied their respective messages with message: "An error occurred while sending mail. The mail server responded: 5.7.1 From address not allowed. Please verify that your email address is correct in your account settings and try again." 4) No changes were effected between successful sends and unsuccessful sends with 5.7.1. messages.
Chosen solution
Your ISP may have changed how they want you to send your outgoing e-mails. ISP's don't necessarily tell their customers when they make changes like this.
Find out from your ISP's website, or phone them, to make sure that your e-mail server settings are correct. Be thorough and check both the incoming (IMAP or POP), and outgoing (SMTP).
Thunderbird Menu: Tools: Account Settings
-- It just occurred to me (to late), to ask if the e-mail address you have in the "From" section of your e-mails is a valid e-mail address, and the one your ISP provided you.
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Chosen Solution
Your ISP may have changed how they want you to send your outgoing e-mails. ISP's don't necessarily tell their customers when they make changes like this.
Find out from your ISP's website, or phone them, to make sure that your e-mail server settings are correct. Be thorough and check both the incoming (IMAP or POP), and outgoing (SMTP).
Thunderbird Menu: Tools: Account Settings
-- It just occurred to me (to late), to ask if the e-mail address you have in the "From" section of your e-mails is a valid e-mail address, and the one your ISP provided you.
Modified by user1121639
Found the answer by searching in ISP Web Site two/three days ago and submitted an Amendment stating same, but could not find way to make personal connection or modify my document.
denis medal Jr. San Francisco CA / MIami Beach Fl
@denis-medal If you want help, please ask a question.