Thanks IRazor for letting us know of this beta, changelog below..
CHANGES FROM 5.1 TO 5.1.1.3 (BETA)
SENDING/CHECKING/FILTERING MAIL
Fixed bug where downloading a message with a really long Message-ID: or Date: header on a non-UIDL server could cause a crash.
Fixed bug related to deleting of nicknames from the auto-complete list.
Nested multipart/{alternative,related} boundary strings no longer contain prefix of outer multipart boundary string.
Added support for the Euro symbol and other special characters.
Incoming messages are now transliterated from ISO-8859-15 or UTF-8 to the default Windows character set as appropriate.
New hidden INI setting,
Made Eudora more tolerant of other applications (i.e. virus scanners or backup utilities) interfering with Eudora"s use of files in its spool directory.
IMAP
Fixed bug in IMAP where if the desired part in a multipart/alternative was not the last part, then it would download the wrong part.
New hidden INI setting, x-Eudora-option:IMAPFilterIncoming, which controls whether or not filters are run over IMAP accounts.
ATTACHMENTS
Fixed bug where attachments with long file names would get saved with an 8.3 name.