Outgoing E-mail Syntax Error

I have made several attempts to send an e-mail to an e-mail address in the following format, xxxxxxx.xxxxxx538@yahoo.com, and repeatedly get an error message as follows: "An error occurred while sending mail. The mail server responded: 5.5.2 RCPT TO syntax error. Please check the message recipient and try again."
However, I can send e-mail to the same e-mail address by using a laptop computer I have using the same i.s.p., Thunderbird mail browser, and the same Windstream router but which uses an imap server for the laptop.
The same p.c. which will not send an e-mail to one e-mail address is able to send an e-mail to other addresses with a similar format which includes a period and numbers.
Please advise.
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