When sending emails with attachment from my android phone, some attachments are not sent or become corrupted when sent. This happens in an unpredictable way when sending emails from my gmail or my gmx account, using the gmail app, the K-9 Mail app or web mail. File size is below 3MB, file type is jpg, mp4 or pdf. It happens on my S5 and my A5 (2017), though less frequently on the latter one. Corrupted files have about the same size as the original file (~ 2% larger) consist of a significant amount of null characters (but not completely) and are not recognized by the linux file command.
I tried rebooting the phone, deleting app data and reinstalling the apps without success.
Trying to send an email multiple times to important contacts is extremely embarrassing. Any help with this issue is highly appreciated.