thanks for the answer. I did some "chore" with the database: archived old elements and compacted the database, reducing it to a third. Since then, looks like everything is working as expected.
sorry if this has been previously addressed, but I did a quick search and I couldn't find anything close to my issue.
I don't know why but I can't send any email with files attached using Outlook 2013 with an Exchange Activesync account. If I use www.outlook.com, everything works...