My saliva will dry as the sealant dries, within a matter of 5–10 minutes.
Something I mail will take 2–4 days to arrive (if i am lucky) and will be touched by a minimum of 4–5 people, and run through various machines for sorting. Each of these people (and potentially the machines) will impart their own set of germs on the envelope. My saliva’s germs, being the oldest and the driest, will likely be long gone before the recipient gets the envelope.
Finally, when they open the envelope (either by pulling the side of the envelope or using a knife or letter opener) , their susceptible body parts will not be close enough to make germ transmission likely. Do you lick envelopes to OPEN them? I don’t.
Bottom line: no risk.