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Unable to Send Using Alternate Email

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I have 2 email addresses with Comcast linked to the same account. I was always able to send an email from either address using Thunderbird and the dropdown in the "From" box. I now receive an error "Sending of the message failed when using the alternate email address. An error occurred while sending mail. Thunderbird responded: "Sending of the message failed. An error occurred while sending mail. The mail server responded: Please ensure your email address and authenticating user match when sending messages. Please verify that your email address is correct in your account settings and try again." I do not know when this started. It had always worked (YEARS). It works on my iPhone mail - it's just Thunderbird. Any suggestions would help....Thanks

I have 2 email addresses with Comcast linked to the same account. I was always able to send an email from either address using Thunderbird and the dropdown in the "From" box. I now receive an error "Sending of the message failed when using the alternate email address. An error occurred while sending mail. Thunderbird responded: "Sending of the message failed. An error occurred while sending mail. The mail server responded: Please ensure your email address and authenticating user match when sending messages. Please verify that your email address is correct in your account settings and try again." I do not know when this started. It had always worked (YEARS). It works on my iPhone mail - it's just Thunderbird. Any suggestions would help....Thanks

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For two accounts, you must have two SMTP servers. They may be to the same email host, but with different email ids. Give each one unique descriptions so you will be able to easily verify that you are establishing the right one for each account. What you're doing used to work okay, but tighter restrictions being imposed by email providers requires this.