I tried %40 in 3 ways:

First I left the ldb as username@domain.tld and changed the username in outlook to username%40domain.tld; and received:

Got user=[username%40domain.tld domain=[MAIN] workstation=[MAIN] len1=24 len2=24
auth_check_password_send: Checking password for unmapped user [MAIN][username%40domain.tld]@[MAIN]
map_user_info: Mapping user [MAIN][username%40domain.tld] from workstation [MAIN]
auth_check_password_send: mapped user is: [MAIN][username%40domain.tld]@[MAIN]
auth_get_challenge: returning previous challenge by module NTLMSSP callback (NTLM2) (normal)
[0000] 61 40 AE 68 94 C6 0B 8F a@.h....
sam_search_user: Couldn't find user [username%40domain.tld] in samdb, under DC=openchange,DC=local

Second I changed ldb to username%40domain.net and left outlook as username@domain.tld; received:

Got user=[username@domain.tld] domain=[] workstation=[MAIN] len1=24 len2=24
auth_check_password_send: Checking password for unmapped user [][username@domain.tld]@[MAIN]
map_user_info: Mapping user [][username@domain.tld from workstation [MAIN]
auth_check_password_send: mapped user is: [domain.tld][username]@[MAIN]
auth_get_challenge: returning previous challenge by module NTLMSSP callback (NTLM2) (normal)
[0000] E4 18 B4 B0 61 AB 31 7E ....a.1~
sam_search_user: Couldn't find user [username] in samdb, under DC=openchange,DC=local

Third, I changed both outlook and the ldb to username%40domain.tld; and received:

Got user=[username%40domain.tld] domain=[MAIN] workstation=[MAIN] len1=24 len2=24
auth_check_password_send: Checking password for unmapped user [MAIN][username%40domain.tld]@[MAIN]
map_user_info: Mapping user [MAIN][username%40domain.tld] from workstation [MAIN]
auth_check_password_send: mapped user is: [MAIN][username%40domain.tld]@[MAIN]
auth_get_challenge: returning previous challenge by module NTLMSSP callback (NTLM2) (normal)
[0000] F5 4D D7 32 F6 06 65 73 .M.2..es
sam_search_user: Couldn't find user [username%40domain.tld] in samdb, under DC=openchange,DC=local