I've done the following:
1) I deleted the message in Webmail. (A support tech confirmed that it is no longer on the Sonic server, but it keeps on downloading.)
2) I deleted a bunch of messages in my SM Inbox on the chance that it might be rejecting incoming mail and then compacted folders several times.
3) I discovered that I was running Ver. 2.49.3 so I upgraded to 2.49.4 and installed it.
4) I called Support and was told that SeaMonkey is 'on its way out and flaky' and that I should try Thunderbird.
5) I posted on the Mozillazine forums and followed the suggestion that I try ThunderbirdPortable as it is easier to uninstall. I should temporarily configure it to leave messages on the Sonic server so that I could easily download them again.
6) I'm having problems configuring Thunderbird so that my ID/PW combination is accepted for POP3 mail.
7) A Support tech helped me configure IMAP and I was able to get the TB interface to display but Get Messages returned a cryptic error message:
An error occurred with the POP3 mail server. Mail server imap.sonic.net responded:
but no further details are not reported in the error message.
8) I can't find an Edit>Preferences in the TB interface to reassure myself that the Mail settings are correct.
9) I have a question posted on the ThuderbirdPortable forum but decided to come here to see if my experience is 'unique.'
FWIW, two experienced forum members are of the strong opinion that the problem is an ISP problem.