I'm sending this in html format, hoping = the dump=20 lines won't wrap. LOL, also, my original message was too = long, so=20 I've shortened it.. sorry if you all get one of these = twice.

Using the configuration you suggested=20 below, (the original configuration I tried and the one that made = the most=20 sense to me also) I've dumped both sides of the tunnel. Below = is the=20 output. From the data below, it's obvious that any outgoing = packets=20 with a full payload (the payload size is 1460)

i.e. 09:29:05.797718 10.1.2.10.3969 = >=20 129.41.69.137.smtp: . 165:1625(1460) ack 125 win 64116 (DF)

never make it to the tunnel = interface. =20 This is why incoming imap appears to work just fine, but sending mail=20 doesn't.

Again, this seems a bit crazy as my = FORWARD chain=20 specifically allows any traffic to and from eth1 and tun0.

$IPTABLES -A FORWARD -i tun+ -j=20 ACCEPT
$IPTABLES -A = FORWARD -i=20 tap+ -j ACCEPT

$IPTABLES -A FORWARD -i eth1 -j = ACCEPT
$IPTABLES=20 -A FORWARD -m state --state ESTABLISHED,RELATED -j = ACCEPT

tcpdump data:

Firewall on the local side of the = tunnel:=20 (tun0)

09:29:04.986164 10.1.2.10.3969 > = 10.1.1.7.smtp:=20 S 266730469:266730469(0) win 64240 <mss 1460,nop,nop,sackOK>=20 (DF)
09:29:05.031684 10.1.1.7.smtp > 10.1.2.10.3969: S=20 361700781:361700781(0) ack 266730470 win 5840 <mss = 1460,nop,nop,sackOK>=20 (DF)
09:29:05.032048 10.1.2.10.3969 > 10.1.1.7.smtp: . ack 1 win = 64240=20 (DF)
09:29:05.070314 10.1.1.7.smtp > 10.1.2.10.3969: P 1:30(29) = ack 1 win=20 5840 (DF)
09:29:05.071048 10.1.2.10.3969 > 10.1.1.7.smtp: P = 1:15(14) ack=20 30 win 64211 (DF)
09:29:05.116340 10.1.1.7.smtp > 10.1.2.10.3969: = . ack 15=20 win 5840 (DF)
09:29:05.117105 10.1.1.7.smtp > 10.1.2.10.3969: P = 30:53(23)=20 ack 15 win 5840 (DF)
09:29:05.122915 10.1.2.10.3969 > = 10.1.1.7.smtp: P=20 15:50(35) ack 53 win 64188 (DF)
09:29:05.158822 10.1.1.7.smtp >=20 10.1.2.10.3969: P 53:61(8) ack 50 win 5840 (DF)
09:29:05.159389=20 10.1.2.10.3969 > 10.1.1.7.smtp: P 50:81(31) ack 61 win 64180=20 (DF)
09:29:05.199339 10.1.1.7.smtp > 10.1.2.10.3969: P 61:69(8) = ack 81 win=20 5840 (DF)
09:29:05.217269 10.1.2.10.3969 > 10.1.1.7.smtp: P = 81:87(6) ack=20 69 win 64172 (DF)
09:29:05.253722 10.1.1.7.smtp > 10.1.2.10.3969: = P=20 69:82(13) ack 87 win 5840 (DF)
09:29:05.461527 10.1.2.10.3969 >=20 10.1.1.7.smtp: . ack 82 win 64159 (DF)
09:29:05.489089 10.1.1.7.smtp = >=20 10.1.2.10.3969: P 69:82(13) ack 87 win 5840 (DF)
09:29:05.489324=20 10.1.2.10.3969 > 10.1.1.7.smtp: . ack 82 win 64159 = (DF)
09:29:05.637836=20 10.1.2.10.3969 > 10.1.1.7.smtp: P 87:93(6) ack 82 win 64159=20 (DF)
09:29:05.674820 10.1.1.7.smtp > 10.1.2.10.3969: P 82:95(13) = ack 93=20 win 5840 (DF)
09:29:05.675137 10.1.2.10.3969 > 10.1.1.7.smtp: P = 93:128(35)=20 ack 95 win 64146 (DF)
09:29:05.711645 10.1.1.7.smtp > = 10.1.2.10.3969: P=20 95:103(8) ack 128 win 5840 (DF)
09:29:05.712153 10.1.2.10.3969 >=20 10.1.1.7.smtp: P 128:159(31) ack 103 win 64138 (DF)
09:29:05.760162=20 10.1.1.7.smtp > 10.1.2.10.3969: P 103:111(8) ack 159 win 5840=20 (DF)
09:29:05.760485 10.1.2.10.3969 > 10.1.1.7.smtp: P 159:165(6) = ack 111=20 win 64130 (DF)
09:29:05.796928 10.1.1.7.smtp > 10.1.2.10.3969: P=20 111:125(14) ack 165 win 5840 (DF)
09:29:05.798040 10.1.2.10.3969 > = 10.1.1.7.smtp: P 4545:4763(218) ack 125 win 64116 = (DF)
09:29:05.798086=20 10.1.2.10.3969 > 10.1.1.7.smtp: P 4763:4768(5) ack 125 win 64116=20 (DF)
09:29:05.834350 10.1.1.7.smtp > 10.1.2.10.3969: . ack 165 win = 5840=20 <nop,nop,sack sack 1 {4545:4763} > (DF)
09:29:05.846786 = 10.1.1.7.smtp=20 > 10.1.2.10.3969: . ack 165 win 5840 <nop,nop,sack sack 1 = {4545:4768} >=20 (DF)

Remote side of the tunnel: = (tun0)

12:08:10.673521 10.1.2.10.3969 > = 10.1.1.7.smtp:=20 S 266730469:266730469(0) win 64240 <mss 1460,nop,nop,sackOK>=20 (DF)
12:08:10.674685 10.1.1.7.smtp > 10.1.2.10.3969: S=20 361700781:361700781(0) ack 266730470 win 5840 <mss = 1460,nop,nop,sackOK>=20 (DF)
12:08:10.718701 10.1.2.10.3969 > 10.1.1.7.smtp: . ack 1 win = 64240=20 (DF)
12:08:10.722990 10.1.1.7.smtp > 10.1.2.10.3969: P 1:30(29) = ack 1 win=20 5840 (DF)
12:08:10.761777 10.1.2.10.3969 > 10.1.1.7.smtp: P = 1:15(14) ack=20 30 win 64211 (DF)
12:08:10.762026 10.1.1.7.smtp > 10.1.2.10.3969: = . ack 15=20 win 5840 (DF)
12:08:10.762144 10.1.1.7.smtp > 10.1.2.10.3969: P = 30:53(23)=20 ack 15 win 5840 (DF)
12:08:10.809779 10.1.2.10.3969 > = 10.1.1.7.smtp: P=20 15:50(35) ack 53 win 64188 (DF)
12:08:10.810126 10.1.1.7.smtp >=20 10.1.2.10.3969: P 53:61(8) ack 50 win 5840 (DF)
12:08:10.846437=20 10.1.2.10.3969 > 10.1.1.7.smtp: P 50:81(31) ack 61 win 64180=20 (DF)
12:08:10.851162 10.1.1.7.smtp > 10.1.2.10.3969: P 61:69(8) = ack 81 win=20 5840 (DF)
12:08:10.905777 10.1.2.10.3969 > 10.1.1.7.smtp: P = 81:87(6) ack=20 69 win 64172 (DF)
12:08:10.906068 10.1.1.7.smtp > 10.1.2.10.3969: = P=20 69:82(13) ack 87 win 5840 (DF)
12:08:11.139331 10.1.1.7.smtp >=20 10.1.2.10.3969: P 69:82(13) ack 87 win 5840 (DF)
12:08:11.148104=20 10.1.2.10.3969 > 10.1.1.7.smtp: . ack 82 win 64159 = (DF)
12:08:11.177095=20 10.1.2.10.3969 > 10.1.1.7.smtp: . ack 82 win 64159 = (DF)
12:08:11.324853=20 10.1.2.10.3969 > 10.1.1.7.smtp: P 87:93(6) ack 82 win 64159=20 (DF)
12:08:11.325240 10.1.1.7.smtp > 10.1.2.10.3969: P 82:95(13) = ack 93=20 win 5840 (DF)
12:08:11.363484 10.1.2.10.3969 > 10.1.1.7.smtp: P = 93:128(35)=20 ack 95 win 64146 (DF)
12:08:11.363818 10.1.1.7.smtp > = 10.1.2.10.3969: P=20 95:103(8) ack 128 win 5840 (DF)
12:08:11.412066 10.1.2.10.3969 >=20 10.1.1.7.smtp: P 128:159(31) ack 103 win 64138 (DF)
12:08:11.412477=20 10.1.1.7.smtp > 10.1.2.10.3969: P 103:111(8) ack 159 win 5840=20 (DF)
12:08:11.447083 10.1.2.10.3969 > 10.1.1.7.smtp: P 159:165(6) = ack 111=20 win 64130 (DF)
12:08:11.449412 10.1.1.7.smtp > 10.1.2.10.3969: P=20 111:125(14) ack 165 win 5840 (DF)
12:08:11.486836 10.1.2.10.3969 > = 10.1.1.7.smtp: P 4545:4763(218) ack 125 win 64116 (DF)

Here is the interesting = part:

Local side of the tunnel, but the eth1 = (private=20 network) interface:

09:29:04.986096 10.1.2.10.3969 >=20 129.41.69.137.smtp: S 266730469:266730469(0) win 64240 <mss=20 1460,nop,nop,sackOK> (DF)
09:29:05.031723 129.41.69.137.smtp >=20 10.1.2.10.3969: S 361700781:361700781(0) ack 266730470 win 5840 <mss=20 1460,nop,nop,sackOK> (DF)
09:29:05.032012 10.1.2.10.3969 >=20 129.41.69.137.smtp: . ack 1 win 64240 (DF)
09:29:05.070351 = 129.41.69.137.smtp=20 > 10.1.2.10.3969: P 1:30(29) ack 1 win 5840 (DF)
09:29:05.071015=20 10.1.2.10.3969 > 129.41.69.137.smtp: P 1:15(14) ack 30 win 64211=20 (DF)
09:29:05.116376 129.41.69.137.smtp > 10.1.2.10.3969: . ack 15 = win=20 5840 (DF)
09:29:05.117139 129.41.69.137.smtp > 10.1.2.10.3969: P = 30:53(23)=20 ack 15 win 5840 (DF)
09:29:05.122880 10.1.2.10.3969 > = 129.41.69.137.smtp:=20 P 15:50(35) ack 53 win 64188 (DF)
09:29:05.158861 129.41.69.137.smtp = >=20 10.1.2.10.3969: P 53:61(8) ack 50 win 5840 (DF)
09:29:05.159354=20 10.1.2.10.3969 > 129.41.69.137.smtp: P 50:81(31) ack 61 win 64180=20 (DF)
09:29:05.199376 129.41.69.137.smtp > 10.1.2.10.3969: P = 61:69(8) ack=20 81 win 5840 (DF)
09:29:05.217234 10.1.2.10.3969 > = 129.41.69.137.smtp: P=20 81:87(6) ack 69 win 64172 (DF)
09:29:05.253760 129.41.69.137.smtp = >=20 10.1.2.10.3969: P 69:82(13) ack 87 win 5840 (DF)
09:29:05.461468=20 10.1.2.10.3969 > 129.41.69.137.smtp: . ack 82 win 64159=20 (DF)
09:29:05.489124 129.41.69.137.smtp > 10.1.2.10.3969: P = 69:82(13) ack=20 87 win 5840 (DF)
09:29:05.489289 10.1.2.10.3969 > = 129.41.69.137.smtp: .=20 ack 82 win 64159 (DF)
09:29:05.637796 10.1.2.10.3969 > = 129.41.69.137.smtp:=20 P 87:93(6) ack 82 win 64159 (DF)
09:29:05.674856 129.41.69.137.smtp = >=20 10.1.2.10.3969: P 82:95(13) ack 93 win 5840 (DF)
09:29:05.675104=20 10.1.2.10.3969 > 129.41.69.137.smtp: P 93:128(35) ack 95 win 64146=20 (DF)
09:29:05.711681 129.41.69.137.smtp > 10.1.2.10.3969: P = 95:103(8) ack=20 128 win 5840 (DF)
09:29:05.712122 10.1.2.10.3969 > = 129.41.69.137.smtp: P=20 128:159(31) ack 103 win 64138 (DF)
09:29:05.760198 129.41.69.137.smtp = >=20 10.1.2.10.3969: P 103:111(8) ack 159 win 5840 (DF)
09:29:05.760453=20 10.1.2.10.3969 > 129.41.69.137.smtp: P 159:165(6) ack 111 win 64130=20 (DF)
09:29:05.796963 129.41.69.137.smtp > 10.1.2.10.3969: P = 111:125(14)=20 ack 165 win 5840 (DF)
09:29:05.797718 10.1.2.10.3969 > = 129.41.69.137.smtp:=20 . 165:1625(1460) ack 125 win 64116 (DF)
09:29:05.797843 = 10.1.2.10.3969 >=20 129.41.69.137.smtp: . 1625:3085(1460) ack 125 win 64116 = (DF)
09:29:05.797966=20 10.1.2.10.3969 > 129.41.69.137.smtp: . 3085:4545(1460) ack 125 win = 64116=20 (DF)
09:29:05.797994 10.1.2.10.3969 > 129.41.69.137.smtp: P = 4545:4763(218)=20 ack 125 win 64116 (DF)
09:29:05.798031 10.1.2.10.3969 >=20 129.41.69.137.smtp: P 4763:4768(5) ack 125 win 64116 = (DF)
09:29:05.834387=20 129.41.69.137.smtp > 10.1.2.10.3969: . ack 165 win 5840 = <nop,nop,sack sack=20 1 {4545:4763} > (DF)
09:29:05.846822 129.41.69.137.smtp >=20 10.1.2.10.3969: . ack 165 win 5840 <nop,nop,sack sack 1 {4545:4768} = >=20 (DF)
09:29:05.847323 10.1.2.10.3969 > 129.41.69.137.smtp: . = 165:1625(1460)=20 ack 125 win 64116 (DF)
09:29:05.847445 10.1.2.10.3969 >=20 129.41.69.137.smtp: . 1625:3085(1460) ack 125 win 64116 = (DF)
09:29:05.847568=20 10.1.2.10.3969 > 129.41.69.137.smtp: . 3085:4545(1460) ack 125 win = 64116=20 (DF)
09:29:06.555338 10.1.2.10.3969 > 129.41.69.137.smtp: . = 165:1625(1460)=20 ack 125 win 64116 (DF)
09:29:08.195721 10.1.2.10.3969 >=20 129.41.69.137.smtp: . 165:1625(1460) ack 125 win 64116 = (DF)
09:29:11.367205=20 10.1.2.10.3969 > 129.41.69.137.smtp: . 165:1625(1460) ack 125 win = 64116=20 (DF)
09:29:17.819496 10.1.2.10.3969 > 129.41.69.137.smtp: . = 165:1625(1460)=20 ack 125 win 64116 (DF)
09:29:30.614817 10.1.2.10.3969 >=20 129.41.69.137.smtp: . 165:1625(1460) ack 125 win 64116 = (DF)