So I had this thought that maybe I ought to be able to send Winlink messages via APRS and thinking thusly, I reasoned that someone probably already made that a thing, so I Googled it.
This is Winlink's page on the subject if you want "official" information: https://winlink.org/APRSLink
I experimented using my Yaesu FT5D. I started by sending the message "Login" to WLNK-1. WLNK-1 responded with an interesting password challenge. This password challenge is a clever way that you can send information about your password in the clear without sending your actual password. The challenge is a three digit number, like 314 and you respond with the characters from your password in those positions (in this example the 3rd, followed by the 1st, followed by the 4th), then you add 3 additional characters at random from your password in random order. So, if your password is ABCDEF and you get the challenge 314 you reply with CADFEB or CADEFB or CADBFE, etc. This is probably the trickiest part. Once you've logged in, you're good for 2 hours. According to the instructions on the Winlink website you can send any text at all to initiate the login procedure. I sent "Login" the first time because I'm an overachiever. I also sent "L" (because I forgot I wasn't logged in) and "a" when I was really lazy. I tried to send a blank message, but my radio won't send a blank message.
Once you've cleared the login hurdle, you can do the stuff. The Winlink site lists 13 commands. I have tried 3 of them.
The "L" (list) command: You just send an L, that's it. Winlink responds with a list of recent unread messages. I think you can see up to 5 messages. (You don't get the actual message, I believe you get some or all of the subject line of the message.)
The "R" (read) command: The reply from the L command assigns each message a number. If you send R2 it will reply with the message that was designated as "2" in the list.
The SMS (short message something?): From what I can gather, SMS has little to do with the very popular cellular phone messaging system. It allows sending short one line messages. To succeed, this message needs proper formatting. I really stretched my capabilities by sending the following:
SMS KN4RBS;KB4TAD;KQ4DCP I sent this message via APRS
Notice the lack of spaces between the addresses? I believe that's important. It seems to use the space as the marker that separates the addresses from the message text. I used semicolons to separate addresses because I observed that format on the Winlink PC client, and it worked.
One drawback is that this functions kind of like an old-school POP/SMTP email system. What I mean by that is that outside of the sending device (in this case my HT) there's no record of the message I sent that can be seen on other devices. If I check my Winlink webmail, it doesn't appear there, and I can't download it to my Winlink PC client. Something I would suggest is adding your own address to the send list if you would like a copy that you can see on your other clients.
Well, that's about all I have to say about my experiences accessing the Winlink system via APRS. I hope this information proves useful to someone.