I have several other virtual servers that are on the same subnet that the SMTP servers are on with no problems. The internal interface of the F5 is on this same subnet. Here are the answers to your questions:
1. Yes, I can successfully telnet to the SMTP server from the F5.
2. Yes, when the SMTP health monitors are enabled, the servers are showing as "up" I have also tried disabling the health monitors with no success.
3. Nothing to capture on the internal side.. This is what I see on the external: .. SMTP server is 172.26.101.48, source is 172.26.101.56, VIP is 172.26.137.134
No. Time Source Destination Protocol Info
1 0.000000 172.26.101.56 172.26.137.134 TCP 47127 > smtp [SYN] Seq=0 Win=5840 Len=0 MSS=1460 TSV=3704817064 TSER=0 WS=7
2 0.000011 172.26.137.134 172.26.101.56 TCP smtp > 47127 [SYN, ACK] Seq=0 Ack=1 Win=4380 Len=0 MSS=1460 WS=0 TSV=3253379147 TSER=3704817064
3 0.000285 172.26.101.56 172.26.137.134 TCP 47127 > smtp [ACK] Seq=1 Ack=1 Win=5888 Len=0 TSV=3704817064 TSER=3253379147
4 12.600101 172.26.137.134 172.26.101.56 TCP smtp > 47127 [RST, ACK] Seq=1 Ack=1 Win=4380 Len=0
4. Nothing in /var/log/ltm about this specific problem, and nothing in the individual SMTP server logs either. I don't believe they are seeing the connection attempt.
Thanks!