MTU Blackhole problem?

[R. W.]

I'm using FreeBSD 5.21 with a dialup modem, and I'm having problem 
uploading files and sending emails with attachments through the 
Fastmail.fm mail service - the connections just time-out. Ordinary web 
browsing and short text emails work normally with Fastmail.  File 
uploads to other sites work and file uploads to Fastmail works from 
Windows 98.

I'm guessing that this is a path MTU discovery blackhole, since if I set    
sysctl  net.inet.tcp.sendspace=400  the problem goes away. (I presume 
that net.inet.tcp.path_mtu_discovery: 1  means that path MTU discovery 
is already turned-on by default.)

What the correct way to deal with this? Should I just play around with
net.inet.tcp.sendspace?