Can't send email with .gif in the signature or attachment

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Chupo

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Aug 20, 2022, 9:52:37 PM8/20/22
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I am debugging an SMTP/POP proxy (Email OAuth 2.0 Proxy) and the
testing configuration is:

Computer A:

- static IP address
- Eudora (which is working for more than 15 years)
- Wireshark

Computer B:

- static IP address
- Proxy
- Wireshark

SMTP and POP server in Eudora installed in computer A are instead with
Gmail's servers configured with the IP address of computer B.

Reading emails via POP is working without problems.

Sending emails via SMPT is working only if there isn't an image in the
signature and if there isn't an attachment (the problem remains with
all firewalls and antivirus SW disabled).

However, if I launch the same Eudora from the virtual machine running
in computer B which is connecting via the very same proxy running
outside of the virtual machine then everything works well - even
sending emails with images in the signature or attachments.

By starting Wireshark capture on both computers just before sending an
email containing attachment I can see the last packet is sent from
computer A but never arrives at computer B.

In the case of sending an email with attached test.txt file containing
just one letter "a" the TCP packet that never arrives looks like:

44 61 74 65 3a 20 54 68 75 2c 20 31 38 20 41 75 Date: Thu, 18 Au
67 20 32 30 32 32 20 30 33 3a 31 35 3a 31 39 20 g 2022 03:15:19
2b 30 32 30 30 0d 0a 54 6f 3a 20 xx xx xx xx xx +0200..To: xxxxx
xx xx xx xx 40 79 61 68 6f 6f 2e 63 6f 6d 0d 0a xx...@yahoo.com..
46 72 6f 6d 3a 20 xx xx xx xx xx xx xx xx xx xx From: xxxxxxxxxx
xx xx xx xx xx xx xx xx 40 67 6d 61 69 6c 2e 63 xxxxxxxx@gmail.c
6f 6d 3e 0d 0a 53 75 62 6a 65 63 74 3a 20 54 65 om>..Subject: Te
73 74 20 28 61 74 74 61 63 68 6d 65 6e 74 29 0d st (attachment).
0a 4d 69 6d 65 2d 56 65 72 73 69 6f 6e 3a 20 31 .Mime-Version: 1
2e 30 0d 0a 43 6f 6e 74 65 6e 74 2d 54 79 70 65 .0..Content-Type
3a 20 6d 75 6c 74 69 70 61 72 74 2f 6d 69 78 65 : multipart/mixe
64 3b 0d 0a 09 62 6f 75 6e 64 61 72 79 3d 22 3d d;...boundary="=
3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d ================
3d 3d 3d 3d 5f 33 37 30 30 33 37 37 38 31 3d 3d ====_370037781==
5f 22 0d 0a 0d 0a 2d 2d 3d 3d 3d 3d 3d 3d 3d 3d _"....--========
3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 5f 33 37 =============_37
30 30 33 37 37 38 31 3d 3d 5f 0d 0a 43 6f 6e 74 0037781==_..Cont
65 6e 74 2d 54 79 70 65 3a 20 6d 75 6c 74 69 70 ent-Type: multip
61 72 74 2f 61 6c 74 65 72 6e 61 74 69 76 65 3b art/alternative;
0d 0a 09 62 6f 75 6e 64 61 72 79 3d 22 3d 3d 3d ...boundary="===
3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d ================
3d 3d 5f 33 37 30 30 33 37 37 38 31 3d 3d 2e 41 ==_370037781==.A
4c 54 22 0d 0a 0d 0a 2d 2d 3d 3d 3d 3d 3d 3d 3d LT"....--=======
3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 5f 33 ==============_3
37 30 30 33 37 37 38 31 3d 3d 2e 41 4c 54 0d 0a 70037781==.ALT..
43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 65 Content-Type: te
78 74 2f 70 6c 61 69 6e 3b 20 63 68 61 72 73 65 xt/plain; charse
74 3d 22 75 73 2d 61 73 63 69 69 22 3b 20 66 6f t="us-ascii"; fo
72 6d 61 74 3d 66 6c 6f 77 65 64 0d 0a 0d 0a 54 rmat=flowed....T
68 69 73 20 69 73 20 6d 65 73 73 61 67 65 20 62 his is message b
6f 64 79 2e 0d 0a 0d 0a 2d 2d 3d 3d 3d 3d 3d 3d ody.....--======
3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 5f ===============_
33 37 30 30 33 37 37 38 31 3d 3d 2e 41 4c 54 0d 370037781==.ALT.
0a 43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 .Content-Type: t
65 78 74 2f 68 74 6d 6c 3b 20 63 68 61 72 73 65 ext/html; charse
74 3d 22 75 73 2d 61 73 63 69 69 22 0d 0a 0d 0a t="us-ascii"....
3c 68 74 6d 6c 3e 0d 0a 3c 62 6f 64 79 3e 0d 0a <html>..<body>..
3c 66 6f 6e 74 20 73 69 7a 65 3d 33 3e 54 68 69 <font size=3>Thi
73 20 69 73 20 6d 65 73 73 61 67 65 20 62 6f 64 s is message bod
79 2e 3c 62 72 3e 0d 0a 3c 2f 66 6f 6e 74 3e 3c y.<br>..</font><
2f 62 6f 64 79 3e 0d 0a 3c 2f 68 74 6d 6c 3e 0d /body>..</html>.
0a 0d 0a 2d 2d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d ...--===========
3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 5f 33 37 30 30 33 ==========_37003
37 37 38 31 3d 3d 2e 41 4c 54 2d 2d 0d 0a 0d 0a 7781==.ALT--....
2d 2d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d --==============
3d 3d 3d 3d 3d 3d 3d 5f 33 37 30 30 33 37 37 38 =======_37003778
31 3d 3d 5f 0d 0a 43 6f 6e 74 65 6e 74 2d 54 79 1==_..Content-Ty
70 65 3a 20 74 65 78 74 2f 70 6c 61 69 6e 3b 20 pe: text/plain;
63 68 61 72 73 65 74 3d 22 75 73 2d 61 73 63 69 charset="us-asci
69 22 0d 0a 43 6f 6e 74 65 6e 74 2d 44 69 73 70 i"..Content-Disp
6f 73 69 74 69 6f 6e 3a 20 61 74 74 61 63 68 6d osition: attachm
65 6e 74 3b 20 66 69 6c 65 6e 61 6d 65 3d 22 74 ent; filename="t
65 73 74 2e 74 78 74 22 0d 0a 0d 0a 61 0d 0a 2d est.txt"....a..-
2d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d -===============
3d 3d 3d 3d 3d 3d 5f 33 37 30 30 33 37 37 38 31 ======_370037781
3d 3d 5f 2d 2d 0d 0a 0d 0a 2e 0d 0a ==_--.......

I thought the problem might be caused by multiple 0x0d 0x0a sequences
in the same packet but email containing many CRLFs in a row without
attachment were sent without problems.

Any idea why the very last TCP packet when there is an attachment is
never delivered or how to do further debugging?
--
Chupo
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