Thanks for that. I have added the debug and got the output below.
Sadly it did not help a lot and even confused me
because I got the same messages about SMTP authentication when it was both 'on' and 'off'.
Perhaps the last message about Unexpected EOF on command channel
is the one I sohuld be thinking about.
Any thoughts on this will be welcome!

send_em no attachments
Net::SMTP>>> Net::SMTP(2.29)
Net::SMTP>>> Net::Cmd(2.26)
Net::SMTP>>> Exporter(5.58)
Net::SMTP>>> IO::Socket::INET(1.29)
Net::SMTP>>> IO::Socket(1.29)
Net::SMTP>>> IO::Handle(1.25)
Net::SMTP=GLOB(0x1d66690)<<< 220 smtp807.mail.ird.yahoo.com ESMTP
Net::SMTP=GLOB(0x1d66690)>>> EHLO localhost.localdomain
Net::SMTP=GLOB(0x1d66690)<<< 250-smtp807.mail.ird.yahoo.com
Net::SMTP=GLOB(0x1d66690)<<< 250-AUTH LOGIN PLAIN XYMCOOKIE
Net::SMTP=GLOB(0x1d66690)<<< 250-PIPELINING
Net::SMTP=GLOB(0x1d66690)<<< 250 8BITMIME
send_em smtp <Net::SMTP=GLOB(0x1d66690)>
Net::SMTP=GLOB(0x1d66690)>>> MAIL FROM:<correct e-mial address>
Net::SMTP=GLOB(0x1d66690)<<< 530 authentication required - "Your email could not be sent.</br To fix this you must make a simple change to your email (known as SMTP authentication).
For advice visit www.btyahoo.com/smtp"
send_em A en <0> <The operation completed successfully.
> result of smtp send ma <>
Net::SMTP=GLOB(0x1d66690)>>> RCPT TO:<correct e-mial address>
Net::SMTP: Unexpected EOF on command channel at email-jun09-b.pz line 159
send_em B en <6> <The handle is invalid.
> result of smtp send ma <> to <0>

In reply to Re^2: SMTP Email problems by merrymonk
in thread SMTP Email problems by merrymonk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.