Seems to have resolved itself. All apparently working OK now.

Thank you all for you assistance 

After 30 or so years of trouble free use I suddenly cannot send emails using Pegasus. My provider is xtra.co.nz.

When sending I get the following message: 

Delivery has failed on the enclosed message for the following

reasons reported either by the mail delivery system on the mail

relay host or by the local TCP/IP transport module:

    554 5.7.1 Message rejected due to possible spam 

 The message is a simple yest to myself, with"test" as the only content.  I have spent a fruitless 2 hours on webchat with an Xtra "tech" but they blamed my end.

I can send OK from the webmail site.
I have checked the Send and Receive settings in Pegasus and they are as required by Xtra; however, there is one - under Security > SSL?TLS .. that they do not specify. It is Use SSL?TLS security on this connection. I have had this on "Via direct SSL Connect" since forever. Should I change this.

Any suggestions for fixing this issue appreciated

Thanks 

[quote user="George"]After 30 or so years of trouble free use I suddenly cannot send emails using Pegasus. My provider is xtra.co.nz.[/quote]

Well, George, I'm sure you know that the real surprise here isn't the fact that after 30 years of working flawlessly you encounter your first troubles with Pegasus Mail but the fact that it ran trouble free for such a long time! [;)] IOW: You're welcome to ask for support, of course!

[quote user="George"]I have spent a fruitless 2 hours on webchat with an Xtra "tech" but they blamed my end.[/quote]

Well, this sounds sounds pretty unprofessional to me, they should at least be able to tell you what exactly makes their spam detection system block your message so we don't have to make guesses.

[quote user="George"]I have checked the Send and Receive settings in Pegasus and they are as required by Xtra; however, there is one - under Security > SSL?TLS .. that they do not specify. It is Use SSL?TLS security on this connection. I have had this on "Via direct SSL Connect" since forever. Should I change this.[/quote]

From the error report you got I don't think so and the only way to figure out is by giving it a try since this shouldn't trigger spam detection.

[quote user="George"]Any suggestions for fixing this issue appreciated[/quote]

Like I wrote above we can only make guesses: There was a time in the past when Pegasus Mail messages were blocked just because the were sent by Pegasus Mail. Pegasus Mail adds a header line to outbound messages reading "X-mailer: Pegasus Mail for Windows (version number)" identifying itself as the email's originator (version number replaced by the actual value). This lead David Harris to not issue this line for a couple of years in previous versions anymore, but in former ones it was re-enabled again. Unfortunately I don't know of any way to prevent it from doing so in current versions.

The message appears to get deliver to the smtp server and then rejected so I don't think it is a configuration issue.  There is a discussion here from June '19 about messages being rejected by xtra.co.nz because they contained an email address or URL.

https://helgeklein.com/blog/2013/11/workaround-554-rejected-due-spam-content-sending-e-mail/

Thanks for those. More reading on the forum last night led me to do a copy and paste from the box that pops up  briefly in Pegasus Mail with coded information on the rejection. I think the last line might hold the key, but I do not know where to go from here. The last few lines are "

20:03:38.946: >> 250 2.0.0 MAIL FROM accepted

20:03:38.946: << RCPT TO:<######>

20:03:39.014: >> 250 2.0.0 RCPT TO accepted

20:03:39.014: << DATA

20:03:39.081: >> 354 3.0.0 continue.  finished with "\r\n.\r\n"

20:03:39.263: >> 554 5.7.1 Message rejected due to

possible spam content 

 "

For clarity: I am sending the test email to myself.

[quote user="George"]I think the last line might hold the key, but I do not know where to go from here. The last few lines are "

[...]

20:03:39.263: >> 554 5.7.1 Message rejected due to

possible spam content[/quote]

Not so, this is the exact same infomation you provided in your starter post. What we need to know is what they classify as "possible spam content". And this information ony they can provide, unfortunately ...

Thanks Michael, I had mentally dismissed the bottom line and was actually talking about the 2nd last, as that does say somethng new (to me at least).

20:03:39.081: >> 354 3.0.0 continue.  finished with "\r\n.\r\n"

Thanks Michael, my mistake, I had mentally dismissed the bottom line as telling me nothing. The second last line however does tell me, at least, something - but I don't know what :-)

20:03:39.081: >> 354 3.0.0 continue.  finished with "\r\n.\r\n" 

[quote user="George"]Thanks Michael, my mistake, I had mentally dismissed the bottom line as telling me nothing. The second last line however does tell me, at least, something - but I don't know what :-)

20:03:39.081: >> 354 3.0.0 continue.  finished with "\r\n.\r\n" [/quote]

My research indicates that this line is a normal part of an SMTP conversation.  It still looks to me like the communication between Pegasus Mail and the SMTP server is fine.  You can see the MAIL FROM is accepted, the RCPT TO is accepted, and the DATA is transmitted.  It is at that point that the server rejects the message as possible spam.

Might your FROM: be something xtra.co.nz doesn't expect or perhaps your default REPLY-TO doesn't match the FROM?

As mentioned previously, make sure there aren't any URL's or email addresses in the body of the message, including the signature.

I was searching the Web after a clue and found something related at https://www.geekzone.co.nz/forums.asp?forumid=39&topicid=238164

Problem is that those forum posts are quite old (2018), though what they mention seems very similar to George's comments.