Attempting to add a BCC recipient has recently begin failing in every instance: whether to self or to a second recipient... the original email goes through correctly.
Using Win7, PMv4.73
An example:

220 smtp.mail.yahoo.com ESMTP ready
<< EHLO [nnn.nn.nnn.nn]
250-hermes--production-ne1-7864dcfd54-q5j27 Hello [nnn.nnn.n.nnn] [nnn.nnn.nnn.nn])
250-PIPELINING
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-SIZE 41697280
250 AUTH PLAIN LOGIN XOAUTH2 OAUTHBEARER
<< AUTH LOGIN
334 xxx
<< xxxxxxxxxxxx=
334 xxxxxx
<< xxxxxxxxxxxxxxxxxxxxxx==
235 2.7.0 Authentication successful
<< MAIL FROM: SIZE=1450
250 2.1.0 Sender SENDER@DOMAIN.com OK
<< RCPT TO:<BCC RECIPIENT@DOMAIN.net>
250 2.1.5 Recipient <BCC RECIPIENT@DOMAIN.net> OK
<< DATA
354 Ok Send data ending with <CRLF>.<CRLF>

554 6.6.0 Error sending message for delivery.

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Thos