Too Many Recipients TT502, port 25

I am sick to death of never knowing how many recipints can be in one email and is there a total maximum for a particular time period?? It seems as if there must be. Now I get a message Too many recipients, when there is ONLY ONE.

Last year I could send about 45 per email and easily 20 of these emails at a session. Now it seems as if I can send hardly any ???

Whats  going on ???