.PS 3.5
arrowhead = 18

define disk {
Disk: [
	ellipsewid=ellipsewid/2
	ellipseht=ellipseht/2
	ellipse
	spline down ellipseht*4/3 from last ellipse.w \
		then right ellipsewid \
		then up ellipseht*4/3 to last ellipse.e
]
}

define machine {
	box wid $1.wid+.3 ht $1.ht+.3 dashed at $1
}

up
Sender: [
ellipse "\fBMUA\fP"
ellipse "MSA"
]
machine(Sender)

arrow 3*linewid from Sender.n

right
Server1: [
ellipse "MTA"
] with .s at last arrow.end
machine(Server1)

arrow 2*linewid from Server1.e
box invis ". . ."
arrow 2*linewid

Server2: [
ellipse "MTA"
ellipse "MDA"
arrow linewid/2
disk
move to Disk.s - (0,.05)
]
machine(Server2)

down
arrow <- 3*linewid dashed from Server2.s+(.86,0)
arrow 3*linewid from Server2.s+(.9,0)

Recipient: [
ellipse "MRA"
ellipse "MDA"
move to last ellipse.w
left
arrow linewid/2
disk
arrow <-> linewid/2
#arrow <-> linewid/2 from Disk.w+(0,.03)
#arrow linewid/2 from Disk.w+(0,-.03)
ellipse "\fBMUA\fP"
] with .n at last arrow.end-(.8,0)
machine(Recipient)

line from .5 <Sender.nw,Server1.sw> to .5 <Server2.se,Recipient.ne> dotted
.PE