Hi Stephen,
Mark says that
matsmats-mm3-test1-confirm+d6f77a2572ef55a8b2c4ca0d77de8493fbb55856@mm3.aca-aws.s.uw.edu
is legal, and I think that most servers will accept it, but RFC 5321 sets a limit of 64 octets on mailboxes[1], and your confirmation mailbox is 67 octets. (There are also limits on domain length and total address length.) Note that although the '+' has a special meaning to the final recipient, and it will be delivered to the mailbox "matsmats-mm3-test1-confirm", which is only 26 octets, intermediate MTAs don't know anything about this convention, they just see the 67-octet mailbox.
I suspect that in a modern server it should be possible to configure a larger value. What is the host with IP 172.19.199.2, do you know? Is it under your control?
Footnotes: [1] https://tools.ietf.org/html/rfc5321 4.5.3.1. Size Limits and Minimums There are several objects that have required minimum/maximum sizes. Every implementation MUST be able to receive objects of at least these sizes. Objects larger than these sizes SHOULD be avoided when possible. However, some Internet mail constructs such as encoded X.400 addresses (RFC 2156 [35]) will often require larger objects. Clients MAY attempt to transmit these, but MUST be prepared for a server to reject them if they cannot be handled by it. 4.5.3.1.1. Local-part The maximum total length of a user name or other local-part is 64 octets.