Help with decrypting email from command line

[steven345 at use.startmail.com]

Hi all,

I have sent and received encrypted emails before and decrypted them successfully. However I have received a few emails from someone that I cannot decrypt using my normal gpg2 commands. Normally I receive encrypted attachments, but these new encrypted data are inline to the email body. However I use an email hosting service that if you provide your keypair, they can encrypt/decrypt emails for you, and it works like a charm every time on these emails: they successfully decrypt these emails using the same keys and passphrase I use on the command line. I've tried many different options and I have no clue why I cannot decrypt these emails on the command line. I copy and paste the armored email contents into a file then simply decrypt it. But with these emails I get "Bad session key" every single time. Since my email hoster can decrypt all of these emails, clearly the encrypted data, keys, and passphrase are good. I use the same keypair to encrypt and decrypt other data no problem, so I'm 
 guessing the sender used some other encryption program/protocol my gpg2 doesn't support but my email hoster does?? The email header Content-Type shows

multipart/encrypted;
boundary="=_MailMate_B366C6F9-8EA1-4E58-BE07-33C7D5454060_=";
protocol="application/pgp-encrypted"

cat -v shows no weird characters in anything I've copy-pasted from the body of the email. It's a standard email:

-----BEGIN PGP MESSAGE-----

etc.............
-----END PGP MESSAGE-----


Any help would be greatly appreciated.

Some info: 

#~ $ gpg2 -d -v -v  wed
gpg: armor: BEGIN PGP MESSAGE
# off=0 ctb=8c tag=3 hlen=2 plen=13
:symkey enc packet: version 4, cipher 3, s2k 3, hash 2
	salt 76D51E30746E495F, count 65536 (96)
gpg: CAST5 encrypted data
# off=15 ctb=c9 tag=9 hlen=2 plen=0 partial new-ctb
:encrypted data packet:
	length: unknown
gpg: encrypted with 1 passphrase
gpg: decryption failed: Bad session key


#~ $ gpg2 --version
gpg (GnuPG) 2.1.11
libgcrypt 1.6.5
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: ~/.gnupg
Supported algorithms:
Pubkey: RSA, ELG, DSA, ECDH, ECDSA, EDDSA
Cipher: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH,
        CAMELLIA128, CAMELLIA192, CAMELLIA256
Hash: SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2



I then upgraded:

#~ $ gpg2 --version
gpg (GnuPG) 2.1.18
libgcrypt 1.7.6-beta
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: /home/user/.gnupg
Supported algorithms:
Pubkey: RSA, ELG, DSA, ECDH, ECDSA, EDDSA
Cipher: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH,
        CAMELLIA128, CAMELLIA192, CAMELLIA256
Hash: SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2
#~ $ gpg2 -d -v -v  wed
gpg: armor: BEGIN PGP MESSAGE
# off=0 ctb=8c tag=3 hlen=2 plen=13
:symkey enc packet: version 4, cipher 3, s2k 3, hash 2
	salt 76D51E30746E495F, count 65536 (96)
gpg: CAST5 encrypted data
gpg: no running gpg-agent - starting '/usr/bin/gpg-agent'
gpg: waiting for the agent to come up ... (5s)
gpg: connection to agent established
gpg: pinentry launched (7500 unknown 0.9.7 ? ? ?)
# off=15 ctb=c9 tag=9 hlen=2 plen=0 partial new-ctb
:encrypted data packet:
	length: unknown
gpg: encrypted with 1 passphrase
gpg: pinentry launched (7515 unknown 0.9.7 ? ? ?)
gpg: decryption failed: Bad session key