I also tried a third, IMAP-focused address I used to use (fastmail), and had the same problem at that address too. FWIW, the message header for the Exchange address includes the following error:
X-Amavis-Alert: BAD HEADER SECTION, MIME error: error: couldn't parse head;
error near:;
UklGRiQxCwBXQVZFZm10IBAAAAABAAEAQB8AAIA+AAACABAAZG F0YQAxCwCUAOQA0P+I/1gA+P+o;
/yAASADo/+D/QAA4ANj/fP98/4j/sP/o/9D/sP84AKQAWACg/3z/+P+w//D/JAHkAPj/8P9YAGgA;
MADw/5j/0P/4/8j/CAA4AMj/wP/g/3z/2P8gAJj/0P94AAgAbP8YA[...]
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