It seems that when mentioning someone with the "@mention" notation the email is not being sent to the mentioned person if the comment is too long (420 characters + mention is sent, 450 characters + m...
I ran several other tests and got the same results. My test which did not get through was e.g.
01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 01234567890123456789012345678 D2/ET2-x2 AKKA MBTEC - Sery, David
Maybe it is also dependant on the length of the mentioned name so you might need to prepend it with more characters. Does it work even when you use e.g. 500 characters?