Outgoing mail can be placed in the mail recepticle where you receive incoming mail. Often the letters get wet. The carrier takes the outgoing letters.

If it's raining the wet letters get tossed into a bucket, then into a larger hamper than into a machine for sorting.

Wet letters make other dry letters wet. So even if you take them to the post office they may encounter wet letters in their way.

Ink runs.

I think the implication was that in Ireland, it's never not raining.

Lol. Man am I stupid.

Although I have had Canadians tell me that their postman does not pick up outgoing mail. Thats what I thought. DUH!2